On estimate of first solar proton arrival

Alexei Struminsky1,2 and Ivan Zimovets1

1 Space Research Institute, Moscow, Russia;

2 IZMIRAN, Troitsk, Moscow region, Russia

Introduction

• Analyzing SEP events it is very important to estimate accurately the time of first relativistic proton arrival to the Earth. Arbitrary a GLE onset observed by the NM network is considered as this time moment. A typical time resolution of NM’s with reasonable statistic accuracy is +/- one minute.

• This accuracy seems reasonable for estimates of first solar proton release into the interplanetary space since free parameters of propagation models, a length of magnetic field line and mean free path, will give a similar error. Depending on solar wind velocity a length of magnetic field line may vary by several tens of % and mean free path is within the Palmer range.

• However this definition of first solar proton arrival does not consider an initial background of NM and a rate of proton intensity increase. These factors may create a larger error, which will lead to incorrect estimate of proton release and, consequently, moment of their acceleration.

• Do we have any instrument with less background level and better statistics? Yes, this is the Anti-Coincidence System (ACS) of Spectrometer on INTEGRAL (SPI)

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• The Anti-Coincidence System (ACS) of Spectrometer on INTEGRAL (SPI) is sensitive to primary and secondary gamma-rays and effectively response to solar energetic particles.

• The ACS SPI provides unprecedented statistics and time-resolution in comparison with NM’s.

• Therefore the ACS SPI may response to solar protons sometimes earlier than do NM’s, if a flux of primary gamma-rays is rather low.

December 13, 2006

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• Is ACS SPI sensitive to relativistic electrons?

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• No, it’s sensitive to relativistic protons, which intensity is lower than NM threshold!

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DISCUSSION

• The proton event of 2006 December 13 began at 02:39 UT (<17 min) by ACS SPI, at 02:50 UT (28 min) by NM’s and at 02:54 UT (32 min) by MEPhI muon hodoscope data.

• Is this difference of the proton event onset caused by a level of background or determined by a threshold energy of the detectors?

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• The same situation was observed for relativistic electrons by STEREO. The onset of electrons with higher energy is delayed.

• This delay is a result of different background.

CONCLUSIONS• An onset of anisotropic GLE phase does not correspond

always to arrival of first solar protons to the Earth. Determining a moment of first solar proton arrival one needs to consider a detector background and a rate of proton intensity increase. An error caused by these factors may be greater than a data time resolution.

• During the 2006 December 13 event, when the intensity of primary gamma-rays was rather low, a massive gamma-ray space born detector (ACS SPI) appeared to be a more effective instrument for observation of the proton event onset than the NM network. The proton event onset was observed by the ACS SPI about 11 min earlier than the GLE onset.

• For two other considered GLE events, when a level of primary gamma-rays was rather high, an arrival of first solar protons was observed by ACS SPI simultaneously with NM’s (2005 January 20) and later (2003 October 2003).

We thank Alexander Lutovinov (Russian “INTEGRAL” data center) for ACS SPI data and all our colleagues submitted NM data to the IZMIRAN data base. IZ thanks the Russian foundation for Basic Research and the Leading Scientific School Program for the financial support.