spicules and prominences: their life...

Mem. S.A.It. Vol. 81, 673c© SAIt 2010 Memorie della

Spicules and prominences: their life together

O. Panasenco

Helio Research, La Crescenta, CA 91214, USA, e-mail: [email protected]

Abstract. Spicules (fibrils against the disk) and filament channels are fundamental partsof the solar chromosphere. What happens to fibrils that are captured in a filament chan-nel? It has been suggested that ”fibrils are associated with spicules” and that filamentsand fibrils are different in nature. However, those fibrils and filaments living together inthe same filament channel have to follow the same magnetic topology rules. This allowsus to trace the structure of the magnetic field in the filament channel at the chromo-spheric level. Spicules/fibrils inside a filament channel represent a basement structure ofthe whole building of a filament and filament cavity above. Therefore, understanding themagnetic pattern of spicules/fibrils in the filament environment will help construct correctfilament/prominence models and resolve some old filament puzzles, such as the bright rimobserved near the feet of filaments/prominences.

Key words. Sun: chromosphere – Sun: corona – Sun: filaments – Sun: magnetic fields –Sun: photosphere – Sun: prominences

1. Introduction

Spicules and prominences, fibrils and fila-ments when observed against the disk, arevery dynamical chromospheric and coronalfeatures, which trace the topology of solarchromospheric and coronal magnetic fields.Howard & Harvey (1964), after studying fila-ments and fibrils in the blue and red wings ofthe Hα spectral line, found that filaments lie athigher layers in the chromosphere than do fib-rils, and they are different in nature (see Fig. 1for the height scales). They also concludedthat fibrils are closely associated with spiculesseen at the solar limb (see also Cragg et al.1963; Beckers 1963). Since spicules are moreor less vertical structures, it is not likely thatthe entire fibril would be seen at the limb asa spicule. Also the lifetime of fibrils is longer

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than spicules, a fact which may be explainedby a difference in their inclination to the verti-cal direction.

It is well known now that fibrils andspicules are different manifestations of thesame chromospheric features emanating fromsupergranulation boundaries and tracing mag-netic field lines. Suematsu et al. (1995) ana-lyzed spicule lifetime and trajectories. UsingDoppler images they found that spicules havean upward radial velocity during the exten-sion phase and a downward velocity duringcontraction, i.e., spicules are truly moving upand down. Suematsu et al. also found a pos-itive correlation of spicule lifetime with theirprojected length, which can be explained bysmaller decelerations for spicules tilted moreto the vertical direction.

The reduction of the contrast between fil-ament and chromosphere when we observe in

674 Panasenco: Spicules and prominences

Fig. 1. Prominences and spicules observed by the Hinode SOT in the Ca II H spectral line on 2006November 30 (left panel) and 2007 August 16 (right panel).

either wing of Hα and, oppositely, an increaseof the contrast for fibrils (see Fig. 2) are ex-plained by the mainly horizontal direction ofplasma motions and magnetic fields inside thefilaments and by the close to vertical rising andfalling of material in the fibrils. When we ob-serve in the wings of Hα the filament becomesvery faint and transparent and we can see thecontrast of fibrils located directly under thefilament. These specific fibrils located in thevicinity of the filament always have a specialpattern: they are parallel or close to parallel tothe filament threads. In the same time the fib-rils located far from the filament have the reg-ular round pattern of a ’rosette’ without spe-cial preferences in any direction. Why does thepattern of fibrils change with distance from thefilament?

2. Filament channel

The answer is known. All filaments and promi-nences are located above places where themagnetic field changes polarity, places knownas a neutral line, a polarity inversion line ora polarity reversal boundary. We also knowthat filaments and prominences originate fromspecific polarity reversal boundaries that arealso inside filament channels (Martin 1990;Gaizauskas 1998) and only appear when fil-ament channels have reached maximum de-velopment; i.e., have reached maximum mag-netic shear along the polarity boundary (Martin

Fig. 2. A dextral filament seen in the core of the Hαline (top) and fibrils in the filament channel 0.5 Åseen in the blue wing (bottom) (from Martin et al.2008).

1998). Not every polarity reversal boundaryis a filament channel: a vital precondition forthe formation of a filament channel is a mag-

Panasenco: Spicules and prominences 675

Fig. 3. Fibrils aligned along a polarity reversalboundary inside the filament channel tracing thehorizontal component of the filament channel mag-netic fields (from Foukal 1971).

netic field having a strong horizontal compo-nent (Gaizauskas et al. 2001).

The formation of specific filament channelsin the chromosphere has been described bySmith (1968), Foukal (1971), Martin (1990),Gaizauskas et al. (1997), Wang & Muglach(2007) and Martin et al. (2008). In the chromo-sphere a filament channel may be recognizedby the presence of fibrils aligned along thepolarity reversal boundary (Fig. 3). This isequivalent to saying that in these locationsthe vector magnetic field is aligned to thefibrils and to the polarity boundary. Since nofibrils cross the polarity reversal line in thefilament channel, the same is true for magneticfield lines, i.e., no magnetic field lines fromactive region or network magnetic fields crossthis boundary in the chromosphere, above orwithin filament channels.

As deduced from chromospheric fibrils,the magnetic field direction smoothly changesfrom upward on one side of a polarity rever-sal boundary to horizontal along the polarityreversal boundary and to vertically downwardon the other side of the polarity boundary.Chromospheric fibrils trace the strong horizon-tal magnetic field component in the filamentchannel (Gaizauskas 1998).

3. Dynamics of fibrils and spicules

Spicules and fibrils are one of the best knownexamples of fine-scale chromospheric dynamic

activity. They are different manifestations (orcounterparts) of the same chromospheric fea-tures emanating from supergranulation bound-aries and tracing magnetic field lines. Themass flux in spicules is about two orders ofmagnitude greater than required for the so-lar wind (Athay 1976), therefore it is im-portant to understand where the plasma ofspicules and fibrils comes from and goes tosince it does not end up in the solar wind.It is important to remember that Hα fib-rils and spicules are not loops. They arerooted at one end in strong magnetic fieldsbut do not have an apparent connection to acorrespondingly strong flux concentration ofopposite magnetic polarity (Gaizauskas et al.1994). Suematsu et al. (1995) found that fib-rils/spicules show true material motions wellrepresented by a parabolic shape on height-time diagrams that can be traced through theup and down phases. Also they found a directrelation between the lifetimes of spicules andtheir maximum lengths, which fits with the factthat the spicular plasma has smaller deceler-ation and falls more slowly in spicules morestrongly inclined to the vertical, and hence tothe direction of the gravitational field.

These results were observationally con-firmed and expanded in the recent papersby Hansteen et al. (2006) and De Pontieu et al.(2007), which also included numerical sim-ulations of motions in the chromosphere.Hansteen et al. (2006) also show that most ac-tive region fibrils observed in the Hα line corefollow quasi perfect parabolic paths, with thevelocity of the fibril top decreasing linearlywith time until it reaches a maximum down-ward velocity that is roughly equal in ampli-tude to the initial upward velocity. The aver-age projected deceleration ranges from 20 to160 m s−2, averaging about 73 m s−2, signifi-cantly smaller than the (downward) solar grav-itational acceleration of 274 m s−2.

The propagating slow-mode magnetoa-coustic shocks found in the two-dimensionalnumerical simulations of Hansteen et al.(2006) had features which fit these propertieswell. The shocks form when waves generatedby convective flows and global p-mode os-cillations in the lower lying photosphere leak


Fig. 4. Sketch of the filament channel magneticstructure: top view (upper panel) and perspectiveview (bottom panel). This magnetic structure con-sists of the filament spine, barbs and fibrils insidea filament channel and overlying arcade magneticfields (notice that fibrils are not part of a filament,neither spine nor barbs, but they follow the topologyof filament channel fields). (For the online color ver-sion: green lines represent the lower part of the fila-ment spine, barbs and fibrils inside a filament chan-nel ; red lines represent the filament spine as usuallyobserved in 304 Å, higher than that observed in theHα spectral line (green lines); blue lines representoverlying arcade magnetic fields).

upward into the magnetized chromosphere.The deceleration in these shock waves dependson the component of solar gravity that is paral-lel to the magnetic field lines along which theshocks propagate. However, it also criticallydepends on the period and amplitude of theshock waves. Even for vertical propagation,

the Hansteen et al. (2006) simulations showthat the deceleration is less than solar gravity.

De Pontieu et al. (2007) concluded that fib-rils follow a parabolic path; in dense plage re-gions they are shorter, undergo larger decelera-tion, and live shorter; fibrils in regions adjacentto dense plage regions are longer, undergo lessdeceleration, and live longer.

So what happens to fibrils that are capturedin a filament channel? We combine everythingthat has been learned about fibrils/spiculesstructure and dynamics above, and frame itwithin a filament channel topology (Fig. 4).

4. Fibrils and spicules inside filamentchannel

First, I would like to summarize some dynamicproperties of the fibrils/spicules plasma drivenby the magneto-acoustic waves (with the wavefronts perpendicular to the magnetic field vec-tor, i.e., k ⊥ B) from the photosphere up to thechromosphere:

1. Plasma is going up and down insidespicules/fibrils.

2. With increasing inclination of thespicules/fibrils from the vertical direc-tion, the plasma propagates along the fieldlines more easily.

3. Deceleration is slower insidespicules/fibrils with a bigger inclina-tion from the vertical.

4. The source of mass is still the same at thebase of spicules and produces mass im-pulses every 3-5 min.

As a result, we have an increasing amount ofmass inside spicules/fibrils with less verticalfields. Such spicules have also longer lengthsand lifetimes.

Second, fibrils/spicules captured in a fila-ment channel can have any angle of inclina-tion from the vertical: spicules can range fromapproximately horizontal to vertical. A deci-sive factor for the geometry of captured fib-rils/spicules is filament height. If a filament istall and the extension between the photosphereand the bottom of the filament spine is equalor bigger than the average height of spicules,


Fig. 5. Why the filling factor f may increase up to ∼ 10 times: left, ”standard” spicule expansion; right,constrained expansion in the neighborhood of a filament.

spicules located under this filament will notfeel the presence of the filament above. Butwith decreasing filament height, spicules willfeel the magnetic field deviating strongly fromthe vertical, and their inclination from the ver-tical direction will increase, causing a conver-gence and squeezing below the filament. Theamount of mass inside these spicules and theirlifetime and length will also increase. As a re-sult the density of plasma will increase, andthis will be observed in Hα as a brightening.

Different factors, from filament height tothe changing density of the fibril/spicule dis-tribution inside a filament channel can lead toan increase or decrease of brightness in theHα spectral line under the observed filament.An increase in brightness may result not onlyfrom a density increase in strongly inclinedspicules in the filament channel, but also fromthe volume available for the spicule jets to ex-haust. Fibrils outside a filament channel have a’rosette’ pattern, with ’leaves’ stretching out-side in any direction filling up a hemisphere2π steradians (Fig. 5, left); but fibrils capturedin a filament channel form a rosette deformedby a strong ’magnetic wind’: the channel mag-netic field. The fibril magnetic fields bend intothe channel field below the filament, thereforethe volume filling factor for fibrils increases

not only because of the reduced vertical ex-tent available, but also because in the horizon-tal plane the 360◦ freedom of the ’rosette’ isreduced to an approximately 20◦ comet likestructure (Fig. 5 right). Figure 5 illustrates howimportant this volumetric effect is: the fillingfactor for fibrils captured in a filament channelincreases at least 10 times.

5. Bright rim

In fact, brightenings under filaments are of-ten observed, and known as bright rims (BRs).These narrow bright bands in Hα located un-der filaments and prominences are especiallywell visible when filaments are located closeto the solar limb and have remained a mysteryfor many decades. Early observations and de-scriptions of bright rims were first provided byRoyds (1920), D’Azambuja (1948), and laterby Gurtovenko & Rahubovsky (1963).

5.1. Main properties of the BRs fromprevious observations

1. Maximum contrast of the rim relative to theundisturbed chromosphere amounts to 1.5.

2. The rim is not observed in filaments withheights h & 10 000 km.


3. The rim is situated in the chromosphere un-der the filament.

4. BRs consist of many distinctly separated,individual brightenings (chains) whichchange their brightness and shape in time.

5. The Hα line emitted by the BR elementsis very similar to the profiles emitted bythe ordinary chromospheric brightenings,such as bright mottles of the quiet chromo-spheric network, for example.

6. BRs exist for all types of filaments(Hansen et al. 1999).

7. BRs are part of the adjacent chromo-sphere rather than a part of the promi-nence/filament (Osarczuk & Rudawy2007).

5.2. Previous models for BRs

Several hypotheses have been suggested to ex-plain BRs:

1. Radiative interaction between the promi-nence and the underlying chromo-sphere (prominence blanketing effect) byKostik & Orlova (1975).

2. Local heating due to magnetic recon-nection (Engvold 1988; Gaizauskas et al.1994; Kononovich et al. 1994).

3. Pure optical effect occurring as a lackof dark structures in the chromosphericnetwork below the filament (Heinzel et al.1990);

4. BRs could be located at the base of mag-netic structures supporting the filament(Kononovich et al. 1994).

5. Diffusion of the Hα radiation in a 1D slabparallel to the solar surface and irradiatedfrom below, BRs are part of the filaments(Heinzel et al. 1995).

6. Paletou (1997, 1998) contested that theory(item 5 above) by using a 2D model of a fil-ament, proving that the BR cannot be partof the filament.

5.3. Some new properties of BRspresented here

The following properties are reported here forthe first time from extended observations of

BRs collected from many sources (both groundand space):

1. BRs fade and fragment and then com-pletely disappear in filaments (or parts offilaments) which are going to erupt (Fig. 6).

2. If a filament with BR erupted without anynoticeable changes in its BR, it implies thatthe trigger for such eruption comes fromthe outside, for example new emerging fluxinside or on the boundary of the filamentchannel (Wang & Sheeley 1999). In otherwords, the eruption occurs without the slowgradual changes usually seen inside the fil-ament channel and bright rim.

3. Since the BRs are not observed in tall fil-aments, but these filaments still exhibit avery active and long life, BRs can not beexplained by reconnection at the feet of fil-aments.

4. The Hα line emitted by spicules with di-rections close to the horizontal are similarto the profiles emitted by the ordinary chro-mospheric bright fibrils, a finding similar tothat of Heinzel & Schmieder (1994) aboutthe same nature of bright and dark mottles.

5. BRs do not appear in the 304 Å spectralline, probably because their brightness iscaused by the increase of density insidespicules/fibrils captured in a filament chan-nel but not by a significant increase in tem-perature.

Therefore, the most likely explanation is thatBRs consist of spicules captured in the filamentchannel and following the approximately hori-zontal magnetic field topology of the channel.

6. Conclusions

The way filaments are formed through con-vective motion, diffusion and differential ro-tation inside an AR, between decaying ARs,between decaying ARs and polar coronal holearea implies that a number of the convectionsupergranular cell borders occur in the vicinityof the filament channel. These borders are thesource of spicules, which are concentrated onthe edges of convective cells.

We have an increasing amount of mass in-side spicules/fibrils with less vertical fields,


Fig. 6. The bright rim under the vertical (NS) part of the filament, clearly visible in the top images, disap-pears (bottom left) before filament eruption occurs (bottom right).

they have also longer lengths and lifetimes.Such geometry is a property of spicules/fibrilscaptured in a filament channel where they canbe observed as a bright rim caused by the in-crease of their brightness in Hα.

Acknowledgements. It is a great pleasure to thankDrs. A. Tritschler, K. Reardon and H. Uitenbroek fortheir efforts in organizing this amazing Workshop,for their generous hospitality, and for their for-bearance in dealing with the author. Thanks alsoto Marco Velli for reading and discussions on themanuscript.

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