changes in sporadic e parameters observed at juliusruh during a period following solar activity...

Acta Geod. Geoph. Hung., Vol. 39(4), pp. 341-354 (2004)

CHANGES IN SPORADIC E PARAMETERS OBSERVED AT JULIUSRUH DURING A PERIOD FOLLOWING

SOLAR ACTIVITY MAXIMUM

F MARCZ l and P BENCZE l

[Manuscript received March 9, 2004]

Using data of Es parameters obtained at a high mid-latitude station in Germany (Juliusruh; geogr.lat.: 54°38/ ) daily and seasonal variations were derived and certain characteristics of these variations have been analysed during a period of changing solar activity. It has been confirmed that diurnal maxima appearing both in foEs and fbEs are generally higher around solar activity maximum than around the minimum. Seasonal maxima appearing in summer are also higher around solar activity maximum than towards the minimum. The daily variation of hiEs showed two maxima: one in the morning and another in the afternoon. The seasonal maximum of hiEs generally appeared in April or May while the minimum in October or November. All these results confirm earlier findings derived for a station in Hungary (Bekescsaba). The present study particularly intends to show changes in the diurnal variation of Es parameters associated with geomagnetic disturbances. The difference between foEs and fbEs might be used as indicator of the transparency of the Es layer, and this quantity seems to increase on days with strong geomagnetic storms. Analysing the transparency based on the maximum values appearing in the daily variations of foEs and fbEs, a rather high quantity has been derived for stormy days in the year 1971. Actually, in this year, the daily variation of hiEs was also characterised by an extremely high afternoon maximum. The wind-shear theory is generally suitable for explaining the generation of sporadic E layer at mid-latitude. The present results are not inconsistent with the points of this theory.

Keywords: Es parameters; geomagnetic activity; solar activity; wind-shear theory

1. Introduction

Sporadic E parameters (foEs, fbEs and hiEs) measured at the mid-latitude station Bekescsaba (Hungary) were previously analysed for the interval 1982-1989, i.e. during the declining phase of solar activity cycle 21 and in the period with increasing activity of cycle 22. The diurnal and seasonal variations of the Es parameters were especially investigated by taking into account the changes with solar activity, too. Moreover, the difference between foEs and fbEs was considered an indicator of the transparency of the Es layer; hereby foEs represented maximum electron density in patches of an Es layer, fbEs characterised the background electron density (Bencze 1983). Based on results of the study presented by Bencze and Marcz (2002), it seemed reasonable to investigate sporadic E parameters measured at another midlatitude station situated at a somewhat higher latitude than that of Bekescsaba.

IGeodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, H-9400 Sopron, POB 5, Hungary, e-mail: [email protected]@ggki.hu

1217-8977/$ 20.00 ©2004 Akademiai Kiad6, Budapest

342 F MARCZ and P BENCZE

Data of sporadic E parameters (similar to those determined at Bekescsaba) have been at disposal for the station at Juliusruh (Germany; geogr. lat.: 54°38' N, geogr. long.: 13°23' E). For analyses of the present study, the interval 1969-1973 was chosen, i.e. the data covered the time of maximum solar activity in cycle 20, as well as a period of decreasing activity.

At first, the main results of the previous analyses (Bencze and Marcz 2002) based on Bekescsaba data are outlined as follows. Regarding the daily variation, both the foEs and the fbEs parameters showed higher values in day-time than at night. The maxima appeared around 10h LT and the minima around 03h LT. On the basis of a rather early study (Bencze 1970), the appearance of two maxima and consequently two minima could be expected in the diurnal variation of the hiEs parameter. Bencze and Marcz (2002) actually confirmed a morning maximum (around 06-07h LT) and an afternoon maximum (around 16-17h LT), while a main minimum was found around midnight and a secondary one around noon. Certain peculiarities also appeared in the daily variations of the mentioned Es parameters when showing their dependence on solar activity. Thus, the daily maxima in the foEs and fbEs parameters were usually lower in the years around solar activity minimum than in those towards the maximum. In the diurnal variation of the hiEs parameter, both the morning and the afternoon maxima showed changes depending on solar activity: both maxima were higher around solar activity maximum than around the minimum.

In addition to the daily variations, seasonal variations were also traced in the Es parameters measured at Bekescsaba station, as reported by Bencze and Marcz (2002). Summer values of both foEs and fbEs were regularly higher in comparison with those for winter. The seasonal maxima appeared mainly in June and July, while the minima occurred in December or January. A peak in the hiEs parameter particularly appeared in May (seldom in April) and the lowest values were usually found in November. The difference between maximum and minimum height in the seasonal variation of the hiEs parameter showed also a dependence on solar activity. This difference (representing the amplitude of the seasonal variation) was larger around solar activity maximum than around the minimum. Considering the transparency of the Es layer (represented by foEs- fb Es ) , a variation both with season and height was detected in this parameter suggesting a connection with similar changes in wind-shear.

As a consequence, the daily and the seasonal variations of Es parameters seemed to be worthy of further study. In the present case, a station situated in the midlatitude zone, however, at a latitude somewhat higher than that of Bekescsaba has been chosen. A data set determined at the station Juliusruh during the interval 1969-1973 was applied, i.e. this period was taken from an earlier solar cycle than that investigated in the case of Bekescsaba (Bencze and Marcz 2002).

Acta Geod. Geoph. Hung. 39, 2004

CHANGES IN SPORADIC E PARAMETERS 343

2. Data and investigation method

Hourly values of the foEs, fbEs and h'Es parameters were at disposal as initial data. Based on these values, hourly medians have been determined for each month, and these monthly values are averaged for the individual years in order to show possible changes in the averaged daily variations between 1969 and 1973. These analyses were especially aimed at checking a solar activity dependence of the Es parameters at a higher latitude than that of Bekescsaba which was studied earlier (Bencze and Marcz 2002).

For revealing effects on the daily variation of Es parameters due to further changing conditions, the consideration of an additional aspect seemed to be reasonable. Thus, days with appropriately strong geomagnetic disturbances were selected and the daily variations of the foEs, fbEs and h'Es parameters measured on these particular days have also been determined. Comparing these extra daily variations with those derived from the complete data set, we should get ideas on a possible geomagnetic influence, too. In this study, a criterion has been applied for selecting geomagnetic disturbances which proved to be useful in analyses of ionospheric parameters described in several earlier works (e.g. Marcz 1986). A daily Kp-sum equal to 30 or higher than this value was prescribed for selecting rather strong geomagnetic disturbances. On this basis, altogether 182 days were selected out from the interval between 1969 and 1973. Actually, values of Es parameters (foEs, fbEs, h'Es) measured on these days are used for determining particular daily variations. The number of samples was generally larger for the night and morning hours than in the hours around noon. Nevertheless, the data set covering five years is certainly suitable for deriving daily variations of the individual Es parameters characteristic for periods with appropriately strong geomagnetic disturbances.

3. Results and discussion

3.1 Daily and seasonal variations in foEs, fbEs and h' Es

Daily, seasonal and solar cycle variations of different sporadic E parameters will be presented for Juliusruh which is a high mid-latitude station. Comparison of these results with the same temporal variations at another mid-latitude station Bekescsaba (46° 42' N; 21°08' E) may confirm the explanation of the characteristics of the variations.

In the case of Juliusruh, the daily variations of the foEs and fbEs parameters have been determined for each year between 1969 and 1973. As they hardly differ from each other in the individual years, results derived for 1971 are shown in Fig. 1, as an example. The diurnal run of foEs and fbEs remind us that the daily variations of the identical parameters determined for Bekescsaba were rather similar (see Bencze and Marcz 2002). The dawn minima appear around 02-03h LT both in foEs and fbEs, like at Bekescsaba, however, the daily maxima are somewhat shifted (from 10 hour) towards the noon hours.

The daily variations of foEs and fbEs indicate the effect of the variation of solar ionizing radiation with the Sun's zenith angle as it is known in the case of the



daily variation of foE. Though, the development of Es layers is due to the vertical shear of the horizontal wind according to the widely accepted wind-shear theory of mid-latitude sporadic E, both foEs and fbEs follow the variation of the solar zenith angle. Namely, the background electron density (electron density in absence of wind-shear), which is modified by the wind-shear mechanism, changes with the solar zenith angle. As it is known, foEs is greater than foE due to the U x B electric field, which forces ions upwards from the height region, where the neutral wind is of W-E direction and moves ions downwards from that height region, where the neutral wind is directed E-W; the wind-shear is producing turbulence. Such a situation can be due to atmospheric gravity waves of appropriate vertical wavelength. Figure 1 indicates that the wind-shear does practically not change during the day. This means that the mean vertical wave-length of gravity waves does also not change, though the vertical wave-length increases with increasing height because of dissipation effects.

Figure 2 distinctly shows a quite continuous decrease of the daily maximum from high solar activity (1969) to the lower one (1973), and this is valid both for foEs and fbEs. The latter trend was also found at Bekescsaba (Bencze and Marcz 2002). Thus, the present results confirm that sporadic E parameters (determined on the diurnal time scale at different mid-latitude stations) really depend on solar activity.

The lawering of foEs and fbEs associated with the decrease of solar activity proves only the role of solar ionizing radiation in the formation of Es layers by causing the decrease of the background electron density (Rottman 1999). As in the case of the daily variations, it also seems (from the latter figure) that the mean wind-shear does not change with solar activity

For checking the behaviour of the Es parameters on a longer time scale, monthly means of foEs and fbEs were determined for the individual years of the interval 1969-1973. Plotting these calculated monthly values, a seasonal variation could be displayed for each year. In spite of faint deviations found in the individual years, rather common characteristics appeared in the seasonal variations of these Es parameters. Thus, results determined for a single year (in this case 1969) are presented in Fig. 3. Both Es parameters show a maximum in summer (regularly in June) and a minimum in winter (December/January). The greatest difference between foEs and fbEs generally occurred in summer. Considering the maximum in the seasonal variation (particularly appearing in June), a decrease is displayed both for foEs and fbEs towards lower solar activity in Fig. 4. All these results well agree with earlier ones derived for Bekescsaba (Bencze and Marcz 2002).

The seasonal variations of foEs and fbEs hint at the same phenomenon as the daily variations; that is, foEs and fbEs increase with decreasing solar zenith angle advancing from winter to summer. However, in this case the difference between foEs and fbEs changes during the year, it is greatest from June to August. The difference foEs- fbEs is greater in the seasonal variation, than in the daily variation. This behaviour may be attributed to the seasonal variation of the wind-shear being greater in the summer months than in the winter ones. The seasonal variation of foEs- fbEs have already been revealed related to the study of transparency of Es


CHANGES IN SPORADIC E PARAMETERS

101Hz 4 .5 .-----------------------------,-----,

Jullu.ruh + 'oEs

4 ~----------------------------+_~~ 11171 " 'bEs

3.5 r-----------::;~~.:::_:_----L...--___i

3 r-----------~~----~~----------~

2.5~----------~------------~~~------~

2~------~~------------~~~~--~

1 .~----~~----------------------~~~

0.5 ~--------------------------------~

O~~~_L~~_L~~_L~_L~~_L~~~

00010203040506070809101112131415161718192021222324

Hour8 (LT=UT+lhour)

Fig. 1. Mean daily variations of foEs and fbEs at Juliusruh for 1971

101Hz 4.------------------------------.-----.

Jull u.ruh + foEs

*' fbEa

3.~--~~--------------~~~------~

3 ~--------~------~--------~--------~ 1969 70 71 72 73

Y •• re

345

Fig. 2. Changes of maximum values in the daily variations of foEs and fbEs between 1969 and 1973 at Juliusruh

101Hz 4.5 .-------------------------,-----------,

Jullulruh + foEs *' fbEs 4r----------~~~~~

3.5i---------------r-------------------l

3 ~----------~~~~==~~----------~

2.5 ~--------~~----------~~~--------~

21-----~~----------------~~~--_1

1.~~~------------------------------~~

0 .5~----------------------------------~ 111611

O ~~--~--L-~--~--L-~--~--L-~--~

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month.

Fig. 3. Mean seasonal variations of foEs and fbEs at Juliusruh for 1969



layers (Bencze 1983, Bencze and Marcz 2002), thus it is confirmed by the present study, too. As mentioned above, the decrease of maxima of the seasonal variations with decreasing solar activity in Fig. 4 indicates the same phenomenon as it has been found in the case of the daily variations of foEs and fbEs; that is, it is associated with lowering of the solar ionizing radiation with decreasing solar activity (Rottman 1999). It can also be assumed that wind-shear does not significantly change with solar activity. However, it is to be noted that the effect of wind-shear is somewhat greater in the case of the maximum of the seasonal variation, (that is, in the summer months), than in the case of the maximum of the daily variation appearing at noon.

At Juliusruh, the mean daily variation of the hiEs parameter is again characterised by two maxima, as it will be shown by examples given in the following section (Figs 10 and 11). (Results considering the effect of geomagnetic disturbances on Es parameters will also be demonstrated, there.) The occurence time of the main maximum in the morning (around 07-09h LT) is a little later than that found for Bekescsaba (Bencze and Marcz 2002) and the secondary maximum in the afternoon (around 15-17h LT) appears somewhat earlier at Juliusruh. Consequently, two minima also appeared in the diurnal variation. As an example for the seasonal variation of hiEs, that one derived for 1971 is demonstrated in Fig. 5. A maximum around April or May, as well as a minimum around October/November is quite typical, even if slight deviations from the proper run also appear in the individual years. The magnitudes of the extreme values (maxima and minima) did not show regular changes from year to year, i.e. the seasonal variation of hiEs might not depend on solar activity, in this case.

The behaviour of the virtual height of Es layers (hiEs) depends on several conditions. It depends on the height of the wind-shear, as a result of which the Es layer is formed and from which reflections originate as shown by the ionograms. Further, hiEs depends on retardation of sounding radio waves in the underlying ionosphere. This is reduced in that case if the ion density in the Es layer and thus fbEs is significantly greater, than the ion density outside of the stratification. Vertical variation of the recombination associated with the vertical variation of metallic ions of meteoritic origin can also affect hiEs since concentration of metallic ions may also change with altitude (Forbes 2000). As it is known, the long lifetime of Es layers is due to the very small recombination coefficient related to these metallic ions. The seasonal variation of hiEs is similar to the seasonal variation of the total ozone content. Almost all ozone in the atmosphere can be found below the height range of the formation of Es layers; the greatest part is in the stratosphere. Ozone absorbs very strongly the solar electromagnetic radiation in the wave-length band 200-300 nm resulting in heating of the middle atmosphere. Heating causes expansion of the atmosphere and the similar run of the seasonal variations of hiEs and ozone would be due to this process (Bencze 1969). Another explanation ofthe seasonal variation might be the winter anomaly in the D region representing enhanced electron density and thus, increased retardation of radio waves in the winter months.


CHANGES IN SPORADIC E PARAMETERS

MHz 3.5.------------------------------,------,

Julluaruh +foEs

2.5L--------L--------~--------L---------" 1969 70 71 72 73

Year.

347

Fig. 4. Changes of maximum values in the seasonal variations of foEs and fbEs between 1969 and 1973 at Juliusruh

h'Es (km) 140.-------------------------------------.

Jullusruh

130~----------------------------------~

120~--------------~~------------------~

110~------------------------------~~~

100~----------------------------------~

1971 90L__L __ ~ __ L__L __ ~ __ L__L __ ~ __ L__L __ ~

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Months

Fig. 5. Mean seasonal variation of h'Es at Juliusruh for 1971

MHz 4.5

Juliusruh +foEs *fbEs 4

1969 - 1973 3.5

Kpsum>30 3

2.5

2

1.5

0.51---------------------------------------1

o~_L~~_L~~_L~~_L~~_L~~_L~

00010203040506070809101112131415161718192021222324

Hours (LT=UT +1 hour)

Fig. 6. Mean daily variations of foEs and ibEs at Juliusruh on days with strong geomagnetic disturbances selected from the period 1969-1973



3.2 Es parameters depending on geomagnetic activity

As mentioned in Section 2, 182 days with rather strong geomagnetic disturbances were selected and used as events occurring in the five-year-interval investigated. Appropriately averaging all foEs and fbEs hourly values measured on these days, mean diurnal variations have been determined for representing the behaviour of both parameters associated with geomagnetic storms. These diurnal variations are presented in Fig. 6. The runs of the two parameters show maxima around noon and minima in early dawn (02h LT), like in the case when using data without selection, i.e. representing common or normal conditions (see Fig. 1 for 1971). Nevertheless, the difference between foEs and fbEs is generally larger in each hour for the days with strong geomagnetic disturbances than that for normal cases. In previous studies (Bencze 1983, Bencze and Marcz 2002), this difference was considered an indicator of transparency of the Es layer. Consequently, Fig. 6 hints at the fact that the transparency of the Es layer increases in periods of geomagnetic disturbances.

Figure 2 revealed a continuous decrease of the diurnal maximum both in foEs and fbEs from solar activity maximum towards the minimum. Figure 7 has been compiled for demonstrating yearly the maximum occurring in the daily variation of foEs (foEs-max) both for days with geomagnetic storms and in the case of using data of all days (available during the investigated period). Hereafter, latter values (derived in this way) should represent average or normal conditions. This comparison shows that the decrease of the daily foEs-max (from high solar activity to a lower one) is also valid for days with geomagnetic storms. Moreover, the yearly foEs-max values are generally greater in the latter case as compared to values representing normal conditions. The differences between these values (belonging to different conditions) are especially significant in years near to the solar activiy maximum (1969 and 1970). Figure 8 shows the behaviour of the daily fbEs-max values in a similar way as Fig. 7 does it for the foEs-max values. Except one year (1971), the yearly fbEs-max values determined for geomagnetic ally disturbed days are always above those representing normal conditions; the decreasing trend with decreasing solar activity is valid in both cases.

In the preceding paragraphs, the maxima appearing in the daily variations of foEs and fbEs have been analysed by comparing appropriate yearly values considering both normal conditions and days with geomagnetic disturbances. On this basis, particular foEs- fbEs quantities can also be derived for representing the transparency of the Es layer during normal and disturbed conditions. This has been done in Fig. 9, where differences between foEs-max and fbEs-max are plotted for each year from 1969 to 1973. Based on all data, this quantity shows slight changes, however, with the highest value occurring in 1971. For the same year, a rather significant increase of the transparency is indicated by a peak value of the difference foEs-max-fbEs-max when data for days with geomagnetic disturbances have been used. As shown in Fig. 8, 1971 was the very year when the blanketing frequency (fbEs) under disturbed conditions has been somewhat below the value derived for normal conditions; on the contrary, in the remaining years, the normal value is



always smaller than that characteristic for disturbed conditions. The foEs-max parameter determined for days with geomagnetic disturbances does not attain an anomalous value in 1971 (see Fig. 7), thus the strongly decreased fbEs-max value in this year should be responsible for the rather increased transparency detected in Fig. 9.

Concerning the effect of individual geomagnetic disturbances on parameters of Es layers, the enhanced corpuscular radiation hitting the terrestrial environment should be taken into account. The increased electromagnetic radiation associated with geomagnetic disturbances is of only short duration, about 1 hour in the lower ionosphere. Thus, its effect (in comparison with that of enhanced corpuscular radiation) may be neglected from the point of view of sporadic E; especially, if yearly means of hourly values are used in the investigation. Increase of transparency indicated by appropriate maxima in daily variations of foEs and fbEs during geomagnetically disturbed periods might be explained by the increased wind-shear in these periods. The auroral zone is a source of wave like perturbations associated with geomagnetic disturbances (Smith 2000, Emmert et al. 2002). At this time, impulse like enhancements of energetic electrons precipitating from the magnetotail into the auroral zone can increase the electrical conductivity there and this leads to the sudden growth of the intensity of the auroral electrojet. As a result, sudden enhancement of the Joule heating causes a sudden expansion of the auroral zone. Thus, a shock wave starts from the auroral zone propagating equatorwards. This shock wave breaks up, it is decomposed into components of different wave-length (travelling ionospheric disturbances). The wave-length increases in course of the propagation due to dissipation. High mid-latitude stations like Juliusruh are not too far from the auroral zone, thus, there are components of short wave-length, the vertical wave-length of which may produce increased wind-shear.

The quite firm decreasing trend of foEs-max and fbEs-max with diminishing solar activity shown for geomagnetically disturbed periods (Figs 7 and 8) might be explained only to a certain extent by the decrease of the solar electromagnetic radiation. Therefore, another fact should also be taken into account; besides the Sq current system generated by atmospheric tides, Es layers are the only phenomena in the lower ionosphere which are related to dynamics of the upper atmosphere through atmospheric gravity waves and turbulence. Considering geomagnetic ally disturbed days, a rather peculiar fbEs-max value appeared in 1971 (Fig. 8) and the value derived for characterising the Es transparency was also curiously high in this year (Fig. 9). It is known that geomagnetic activity can show two maxima during the quasi ll-year solar cycle. The first maximum may occur before (or simultaneously with) the solar activity maximum, while the second one appears after the solar activity maximum. (As a consequence, a minimum also appears in geomagnetic activity between the two maxima which sometimes just coincides with the solar activity maximum.) This phenomenon is called after its discoverer "Gnyevisev gap". In the present study, a period of solar cycle 20 has been investigated. During this cycle, the year 1971 was characterised by a somewhat decreased geomagnetic activity coinciding with a quite strong decrease in solar activity. A connection between the peculiar values of Es parameters found for 1971 (Figs 8 and 9) and the previ-



foEs-max (MHz) 4.2 I'" normal * Kpsum>30

....-x... 4 ~ ~ Juliusruh

3.8

--------- .. ---3.6

3.4

3.2 1969 1970

, , , , ~ '-'"" ~ ----- .....

" "

'" .:-

1971 1972 1973

Years

Fig. 7. Changes of maximum values in the daily variations of foEs at Juliusruh both for normal and geomagnetically disturbed days between 1969 and 1973

fbEs-max (MHz) 4 I" normal ~ Kpsum>30

3.8

~ Juliusruh

----'----~ ~ ~ ",

3.6

"~---."~ 3.4

3.2

o

o

o

o

o

o

3 1969 1970 1971

Years

, ,

1972 1973

Fig. 8. The same as in Fig. 7, but for fbEs

MHz .6 I'" normal * Kpsum>30 Juliusruh A .5

/ ~ (foEs max - fbEsmax)

.4

/ ~ .3

/ --. ~ _ ..... ------- .......

--- " .~ --- ................ -------.1

0 1969 1970 1971 1972 1973

Years

Fig. 9. Changes of f oEs-max- fbEs- max differences (representing the transparency of the Es-layer) at Juliusruh both for normal and geomagnetically disturbed days between 1969 and 1973



ously mentioned "Gnyevisev gap" might not be excluded, even if further analyses are required for finding the actual mechanism contributing to such a phenomenon.

From the foregoing it is quite clear that an analysis of the daily variation of the virtual height of the Es layer (hiEs) might also be informative. For 1973, the diurnal variation of hiEs is determined both in the case of normal and geomagnetically disturbed conditions, as shown in Fig. 10. The normal curve reveals a higher maximum in the morning than in the afternoon, while an opposite behaviour is shown by the run of the diurnal curve derived for disturbed conditions. For 1971 (in which year both the value of the diurnal maximum of the fbEs parameter and the quantity characterising the Es transparency seemed to be peculiar), a rather high afternoon maximum is detected in the daily variation of hiEs, if data obtained on days with strong geomagnetic disturbances are analysed (see Fig. 11). This afternoon maximum is a rather high peak which represents the main maximum as compared with the slight secondary maximum found in the afternoon for normal conditions. On the contrary, the morning maximum is the main one under normal conditions and its value hardly differs from that derived for disturbed conditions which is only a secondary maximum in these morning hours. All this hints at rather changed conditions in the height of the Es layer during strong geomagnetic disturbances, as compared with the normal cases.

As it has already been mentioned, enhanced gravity wave activity producing wind-shear might also contribute to the generally increased hiEs values in geomagnetically disturbed cases (Smith 2000, Emmert et al. 2002). This fact seems to be valid both for 1971 and 1973, since the mean level of the daily variation of this parameter is really higher on stormy days in comparison with normal days (as shown in Figs 10 and 11). The daily variation of hiEs under geomagnetically disturbed conditions, however, indicates firmly higher values (on an average) for 1971 than for 1973. It might be assumed, that a greater underlying ionization and a larger retardation of sounding radio waves is also responsible for this behaviour in 1971, as compared to 1973. The peculiar appearances of the maxima in the disturbed daily variation of hiEs are also remarkable. Previous investigations have already shown that the morning maximum of the mean daily variation is higher than the afternoon maximum (Bencze 1970). As derived from data used in the present study, the mean daily variation of hiEs on geomagnetically disturbed days indicates a significantly higher maximum in the afternoon than in the morning. The 12 hour component of atmospheric tides seems to playa role in this behaviour, namely this component is distorted by atmospheric gravity waves. In the case of the semidiurnal tidal wind component, the distortion is shown by the departure from the time difference of about 12 hours between the maxima; while here in this study, a similar signature is shown by the time difference between the maxima of curves plotted in Figs 10 and 11.



h'Es (km) 140.---------------------.--------------,

Juliusruh

130~------------------------~--------~

110~~--------------------------~~_..~_~.~

--100~----------------------------------~

1973

90~_L~~~_L~~~_L~~_L_L~~_L~~

00010203040506070809101112131415161718192021222324

Hours (LT=UT +1 hour)

Fig. 10. Mean daily variations of hiEs at Juliusruh both for normal and geomagnetically disturbed days in 1973

h'Es (km) 150.------------------------------------.

Juliusruh

140~--------------------~~._--------~

120~----~~~----------~--~=_~----~

1971 ... normal "* Kpsum>30 100~-L~~~-L~~-L-L~L-------------~

00010203040506070809101112131415161718192021222324

Hours (LT=UT+1hour)

Fig. 11. The same as in Fig. 10, but for 1971

4. Summary

Results of the present study partly confirm earlier ones published by Bencze and Marcz (2002) for a lower mid-latitude station (Bekescsaba), partly yield a new aspect for understanding certain links between different Es parameters observed at mid-latitudes. Comparing results derived for two different stations (Bekescsaba and Juliusruh) and sampling data from two different solar activity cycles, it was found that the diurnal maxima both in foEs and fbEs are generally higher around solar activity maximum than around the minimum. The seasonal variations of foEs and fbEs attain to their maxima in summer (to minima in winter) and these seasonal maxima decrease with decreasing solar activity at both investigated stations. Both a morning and an afternoon maximum has been detected in the diurnal variation of the h'Es parameter. In the individual years, the seasonal variation of h'Es showed generally a maximum in April or May, while the minimum appeared in October or November.

Acta Gead. Geaph. Hung. 39, 2004


The principal aim of the present study was the detection of effects in the diurnal variation of Es parameters associated with geomagnetic disturbances. Therefore, 182 days with rather strong storms (Kp-sum > 30) were selected and Es data obtained on these days have been used for compiling particular diurnal variations. For the interval 1969-1973, the runs of the daily variations of foEs and fbEs parameters are quite parallel to each other considering data measured whether during average (normal) conditions or on geomagnetic ally disturbed days. Nevertheless, the transparency of Es layer, as indicated by the difference between foEs and the blanketing frequency (fbEs) , seems to increase during geomagnetic storms. Analysing this difference on the basis of foEs and fbEs maximum values (appearing in the daily variations of these parameters at Juliusruh, as determined for the individual years), a rather increased transparency might be suggested for the stormy days of the year 1971. It seems that a strangely low fbEs-max value associated with geomagnetic storms might certainly contribute to the quite peculiar behaviour of the transparency in this year.

Considering the daily variation of hIEs, highly increased values appeared in the afternoon on days with strong geomagnetic storms (and they are well above the values determined for normal conditions) as shown both in Figs 10 and 11. This strange afternoon maximum was especially distinct in 1971 when a highly enhanced transparency of the Es layer was also detected (as indicated by the increase of the foEs- fbEs difference in this year). Based on results of the present study, the windshear theory described by Whitehead (1961) might be invoked for explaining the behaviour of certain Es parameters at mid-latitudes, like it was also done in a previous work (Bencze and Marcz 2002).

This theory involves vertical shears (velocity gradients) in horizontal winds in the neutral air at E region heights. It suggests that the ionization in the E region is redistributed by shears and this fact leads to the formation of sporadic layers. In the theory, the vertical motions of ionization are attributed to horizontal wind blowing across the magnetic field. Heisler and Whitehead (1960) found a connection between Es occurrence and the strength of the horizontal geomagnetic field component. Analyses of the present study have revealed important changes in the diurnal variation of the hIEs parameter (obtained at Juliusruh) associated with selected geomagnetic storms. Consequently, these results serve as further contribution hinting at the reality of wind-shears in E region heights, and they (together with earlier findings) confirm that the wind-shear theory is certainly suitable for explaining the generation of sporadic E layers at mid-latitudes, (even under disturbed conditions).

Acknowledgement

This study has been supported by the Hungarian Space Office through the grant TP 121.



References

Beneze P 1969: Pure and applied Geophysics, 75, 167-174. Beneze P 1970: Acta Geod. Geoph. Mont. Hung., 5, 223-231. Beneze P 1983: INAG Bulletin, 40/41, 12. Beneze P, Marez F 2002: Acta Geod. Geoph. Hung., 37, 227-238. Emmert J T, Fejer B G, Shephard G G, Solheim B H 2002: J. Geophys. Res., 107, A12,

SIA 19. Forbes J 2000: J. Atm. Solar- Terr. Phys., 62, 1603-1621. Heisler L H, Whitehead J D 1960: Nature, 187, 676-677. Marez F 1986: Acta Geod. Geoph. Hung., 21, 201-207. Rottman G 1999: J. Atm. Solar- Terr. Phys., 61, 37-44. Smith R W 2000: J. Atm. Solar-Terr. Phys., 62, 1623-1628. Whitehead J D 1961: J. Atm. Terr. Phys., 20, 49-58.


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