Free Access
Volume 531, July 2011
Article Number A46
Number of page(s) 8
Section The Sun
Published online 09 June 2011

Online material

Appendix A: Response of EPAM to solar X-rays

It is well known that energetic particle instruments using solid state detectors experience a response to solar X-rays if they are pointed at the Sun. Therefore it is customary to ignore the data from such detectors at times of intense solar X-ray activity. The LEMS30 detector on the EPAM instrument has two sectors out of four which scan the Sun as the spacecraft spins. The magnetically-deflected electrons enter the instrument through the same aperture as the ions detected by LEMS30, but a magnet sweeps them into a well-shielded detector which has negligible response to solar X-rays. In this appendix we demonstrate the integrity of the deflected electron data by showing the responses of both the electron detector and the LEMS30 ion detector at times of intense solar X-ray activity.

Table A.1

Large X-ray flares which contaminate sunward-facing detectors.

thumbnail Fig. A.1

Contamination in the LEMS30 detector from solar X-rays on 4 November 1997. The upper panel is the P4 channel, which is designed to respond to ions with energies between 193 and 315 keV. The lower panel are the magnetically-deflected electrons, which enter the EPAM instrument through the same aperture as the LEMS30 ions.

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The flares we have chosen are given in Table A.1. They are all GOES X-ray class X-flares, and three of them are among the largest of the solar cycle 23. Figure A.1 shows the intensity-time history from 05:45–06:45 UT on 4 November 1997 for EPAM channnels P4 and DE1, which are designed to measure 193–315 keV ions and 38–53 keV electrons. This flare was GOES class X2.1 and it was preceded by a relatively quiet period, with a low ambient background of charged particles. The data are plotted showing the four sectors independently. The detected ions and electrons enter the instrument though the same aperture, which for two of the sectors (blue and green) sweep past the Sun. It is clear that the P4 channel is responding to solar X-rays in these two sectors, but not in the other two sectors, plotted as red and cyan. The colour code for the electrons is identical. About 20 min later a beam of electrons is detected, which is the normal timing given the longitude of the flare at W33.

The second event we have chosen was GOES class X20, and Fig. A.2 shows the P4 and DE1 channels for the period 21:30–22:30 UT on 2 April, 2001. April 2001 was an active month and the pre-flare background intensity was enhanced over quiet times. Note that the intensities in Fig. A.2 are plotted logarithmically. The P4 channel responds to the flare X-rays in the blue and green sectors. The electron channel does not repond to the solar X-rays and  ~20 min later the electrons start arriving at ACE as an anisotropic beam. This behaviour is normal for particles associated with a flare near the solar west limb.

thumbnail Fig. A.2

As Fig. A.1 except for 2 April 2001.

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thumbnail Fig. A.3

As Fig. A.1 except for 15 April 2001.

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Figure A.3 shows the same channels for the time period 13:30–14:30 UT on 15 April 2001. The detector response to the flare is the same as before. Figure A.4 shows the same channels for the period 10:50–11:50 UT on 28 October 2003. The flare was GOES class 17.2 and we have chosen this event partly because there is a very small electron increase aound the onset of the solar X-ray event. However, this is a short-lived beam of solar electrons which must have left the Sun some 15–20 min before the flare onset. We know that the response is not due to X-rays as the peak sector (red) is not a sector that views the Sun, and the sector with the lowest intensity (cyan) is one of the sectors where the instrument aperture sweeps past the Sun. The 28 October 2003 event has been analysed in depth by Simnett (2005).

thumbnail Fig. A.4

As Fig. A.1 except for 28 October, 2003.

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The final event we present is that occurring on 6 December, 2006. We have chosen this event as (a) it is a GOES X6.5 flare; and (b) it is the day after the event which is the subject of this paper. Figure A.5 shows the data from 18:30–19:30 UT. The response to this event is essentially identical to that shown in the

previous events, with the two sunward sectors of LEMS30 showing an increase above the (enhanced) background but with no increase in either the non-sunward sectors or the electrons.

thumbnail Fig. A.5

As Fig. A.1 except for 2 April 2001, and the upper panel shows the P3 channel of LEMS30, designed to detect ions from 115–193 keV.

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The conclusion we draw from this analysis is that the deflected electron channels in the EPAM instrument are not contaminated by solar X-rays.

© ESO, 2011

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