Solar energetic particle variations

Abstract

In the largest solar energetic-particle (SEP) events, acceleration occurs at shock waves driven out from the Sun by coronal mass ejections (CMEs). In fact, the highest proton intensities directly measured near Earth at energies up to -1 GeV occur at the time of passage of shocks, which arrive about a day after the CMEs leave the Sun. CMEdriven shocks expanding across magnetic fields can fill over half of the heliosphere with SEPs. Proton-generated Alfven waves trap particles near the shock for efficient acceleration but also throttle the intensities at Earth to the "streaming limit" early in the events. At high energies, particles begin to leak from the shock and the spectrum rolls downward to form an energy-spectral "knee" that can vary in energy from -1 MeV to -1 GeV in different events. All of these factors affect the radiation dose as a function of depth and latitude in the Earth's atmosphere and the risk to astronauts and equipment in space. SEP ionization of the polar atmosphere produces nitrates that precipitate to become trapped in the polar ice. Observations of nitrate deposits in ice cores reveal individual large SEP events and extend back -400 years. Unlike sunspots, SEP events follow the -80-100-year Gleissberg cycle rather faithfully and are now at a minimum in that cycle. The largest SEP event in the last 400 years appears to be related to the flare observed by Carrington in 1859, but the probability of SEP events with such large fluences falls off sharply because of the streaming limit. INTRODUCTIONThe solar wind, which blows at speeds of 300 to 800 W s , corresponds to proton energies of 0.5 to 3 keV. The protons in solar energetic-particle (SEP) events have energy spectra that span the region from about 10 keV to >10 GeV. In large SEP events, omni-directional fluences of >30 MeV protons can exceed 10"

Pickup ion measurements by MAVEN: A diagnostic of photochemical oxygen escape from Mars

“…During intense solar particle events, the flux of solar energetic particles associated with solar flares and coronal mass ejections will likely exceed the predicted pickup ion fluxes. However, on average, these solar events only occur a few times a month [Reames, 2004;Jun et al, 2007].…”

“…Assuming that hot O atoms undergo no collisions above 300 km, exospheric densities are calculated using Liouville's theorem [Schunk and Nagy, 2009], where the energy distribution function is fed into a Liouville code to find the hot O number density as a function of altitude in the Martian exosphere out to 100 R M . This is done separately for the bound and escaping portions of the distribution.…”

“…A wealth of evidence gathered from orbital and surface missions indicates that Mars had a much higher abundance of volatiles in its distant past [e.g., Solomon et al, 2005;Fassett and Head, 2011] and likely had at least episodically stable liquid water [e.g., Carr, 1996;Leshin et al, 2013]. A major goal of the NASA MAVEN (Mars Atmosphere and Volatile EvolutioN) mission is to quantify atmospheric loss processes (nasa.gov/maven), and an important loss process is the escape of neutral oxygen generated by ionospheric and thermospheric photochemistry [Chassefière and Leblanc, 2004].…”

Space Weather: The Solar Perspective

“…For the very largest events, SEP ionization of the polar atmosphere produces nitrates that precipitate to become trapped in the Earth's polar ice. Ice core analysis revealed that the largest SEP event in the last 400 years appears to be related to the giant flare observed by Carrington in 1859 (Reames, 2004). Forecasting such extraordinary events is still not possible, for two main reasons: 1) We have not yet identified unique signatures for the driving flare that would indicate an imminent explosion and its probable onset time, location, and strength, and 2) the size of the SEP fluxes is highly variable and appears to be only loosely related to the strength of the flare.…”

“…The time profile of the strong SEP proton flux event of November 4, 2001 (from Reames, 2004) is shown in Figure 22. After a steep flux increase within minutes after flare onset, the fluxes in the low energy bands remain about constant for several hours while for higher energies the fluxes gradually drop, i.e., the energy spectra become steeper.…”