1. Solar Energetic Particle Events Observed by Lunar Prospector

2. Electron Reflectometer Instrument MAG • Top hat electrostatic analyzer • Designed to measure 10 eV – 20 keV electrons • Higher energy particles (>1 MeV electrons, >20 MeV protons) penetrate the housing (bypassing the optics) and are also counted by the ER. • During SEP events, penetrating counts dominate in the highest energy channels of the instrument. ER

3. Event List:Date & GSE Longitude of Moon • 04/20/1998 281-285 • 04/30/1998 49-59 • 05/04/1998 97-105 • 05/06/1998 121-129 • 08/24/1998 22-33 • 09/24/1998 45-53 • 09/26/1998 63-97 • 09/30/1998 109-127 • 11/14/1998 307-312 • 04/21/1999 78-90 • 05/05/1999 235-298

4. Sample Event (Sep. 30, 1998) Count Rate in 15 energy channels from 40 eV (red) to 16 keV (blue). Periodicity at low energies is due to the two hour polar orbit around the Moon, which blocks low energy solar wind flux. Possible magneto-sheath entry

5. Highest Energy Channels (dominated by penetrating flux) Three highest energy channels. Where close, flux is mostly due to penetrating particles. Highest energy channel, separated by look directions, as described on the next slide.

6. Detector Look Directions 2 2 2 2 1 1 Sun 1 1 1 1 1 1 2 2 2 2 Sectors labeled (1) are sometimes contaminated by photons, (2) are not.

7. Highest Energy Channel Again (separated by look direction) Blue = Red + Green Periodicity before event is only in 1 sectors, and is therefore likely due mostly to photons. During event, periodicity is in all sectors, and therefore is due to either electrons or penetrating particles being shadowed by the Moon.

8. Integrated Event Spectrum Black = in front of Moon Pink = behind Moon Solid = 2 sectors Dashed = 1 sectors Mostly low energy electrons Mostly penetrating particles

9. Integrated Event Spectrum Vs. Pre-Event Spectrum Orange = in front of Moon pre-event Blue = behind Moon pre-event Photon Contamination

10. Shadowing? • There is anti-solar shadowing in the highest energy channels of the ER during the event. • It is not due to photons. • Is it shadowing of “real” 16 keV electrons that follow the optics, or is it shadowing of high-energy penetrating particles? • There are way more penetrating particles than there were “real” electrons pre-event, suggesting the latter. But, there may also be more “real” electrons during the event! • In either case, why is the shadowing anti-solar, rather than controlled by magnetic field direction?

11. Event Spectrum – Pre-Event Spectrum Not flat, so clearly there are also more “real” electrons during event than pre-event, as well as the penetrating particles.

12. Event spectrum from in front of Moon – Event spectrum from behind Moon Constant difference at high energies suggests shadowing of penetrating particles rather than “real” electrons. “Real” electrons being shadowed should give a power-law difference as a function of energy (as seen at low energies). Difference in Count Rate Energy (eV)

13. Comparison with WIND EPACT Good comparison In general with WIND, ACE, and GOES. LP ER Ratios suggest LP effective geometric factor for SEPS of ~8-10 cm2 sr, or about a quarter of what one would expect geometrically Arbitrarily Normalized WIND EPACT Time (s)

14. Summary • Many SEP events seen by Lunar Prospector. • Good comparison to other spacecraft. • Anti-solar shadowing seen – still ambiguous whether it’s “real” low-energy electrons or high energy penetrating particles – though there is at least circumstantial evidence that we’re seeing shadowing of penetrating particles.