# Presentation: 701979

## Solar Energetic Particle Events Observed by Lunar Prospector

## Electron Reflectometer Instrument

- 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

- MAG

## 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

## 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

## 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.

## Detector Look Directions

- Sun

- 1

- 1

- 1

- 1

- 1

- 1

- 1

- 1

- 2

- 2

- 2

- 2

- 2

- 2

- 2

- 2

- Sectors labeled (1) are sometimes contaminated by photons, (2) are not.

## Highest Energy Channel Again (separated by look direction)

- 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.

- Blue =
- Red +
- Green

## Integrated Event Spectrum

- Black = in front of Moon
- Pink = behind Moon
- Solid = 2 sectors
- Dashed = 1 sectors

- Mostly low
- energy electrons

- Mostly penetrating
- particles

## Integrated Event Spectrum Vs. Pre-Event Spectrum

- Orange = in front
- of Moon pre-event
- Blue = behind
- Moon pre-event

- Photon
- Contamination

## 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?

## 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.

## 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).

- Energy (eV)

- Difference in Count Rate

## Comparison with WIND EPACT

- Good comparison
- In general with
- WIND, ACE, and
- GOES.

- LP ER

- WIND EPACT

- Arbitrarily Normalized

- Ratios suggest LP
- effective geometric
- factor for SEPS of
- ~8-10 cm2 sr, or
- about a quarter of
- what one would
- expect geometrically

- Time (s)

## 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.