About

The overarching goal of the Imaging Microburst Precipitation with Atmospheric X-ray emissions  (IMPAX) CubeSat is to quantify relativistic electron microburst precipitation as a radiation belt  loss mechanism. The degree to which microbursts deplete the outer radiation belt is a major  outstanding question in radiation belt physics that has significant implications for manned space  flight and space-borne instrumentation including GPS technology, as well as dramatically  affecting atmospheric chemistry. Microbursts are highly localized sub-second precipitation  events of keV-MeV electrons. They are observed at low altitude on magnetic field lines mapping  to the outer radiation belt, most often during the main phase and early recovery phase of  geomagnetic storms. They are known to be a significant loss mechanism for outer belt electrons  during storm time, but the importance of microburst precipitation relative to other radiation belt  loss processes has not been established. We propose a mission to make this critical quantification  by answering the following Science Questions:  

1. What is the energy distribution (flux and spectrum) of electron microburst  precipitation into the Earth’s atmosphere? 

2. Where are relativistic electron microbursts that precipitate into the Earth’s  atmosphere generated? 

The answers to these questions will enable us to quantify this loss process to a much greater  extent than either previous measurements or upcoming funded missions allow. The energy  distribution will provide the flux of electrons at different energies precipitating within individual  microbursts, trains of microbursts, and over the full precipitation region, as well as how this flux  varies temporally (storms/substorms) and spatially (L/MLT). Identifying the generation location  of microbursts as well as properties (location, emission characteristics) of the causative chorus will significantly constrain the spatial and temporal scale of microburst precipitation. IMPAX  will determine how this generation location varies with microburst energy, for storms or  substorms, and by L, MLT, or MLAT of the observed precipitation. Combining the measured  energy distribution of precipitating electrons with the spatial and temporal scale of the  precipitation region will determine how long it would take for microburst loss to deplete the  outer radiation belt. This timescale will then be compared to the observed energy-dependent  outer belt decay, which will quantify the loss that can be accounted for by microburst  precipitation and thus its relative importance as a radiation belt loss process. 

To answer these questions IMPAX will take the novel approach of combining high  resolution direct measurement of electrons within or near the loss cone and the first ever  imaging in low-earth-orbit (LEO) of bremsstrahlung X-rays created when precipitating  microbursts hit the atmosphere. The combination of these instruments allows a precise  measurement of the bounce loss cone electron flux and spectrum that is not possible with only  direct electron measurements at LEO, or short-duration balloon measurements of X-rays at low  altitude. The ultra-high energy resolution particle measurements will also show the energy time dispersion that will allow identification of the microburst generation location. The  innovative I-AXIS atmospheric imager will be able to image these X-ray emissions over a significant segment of the precipitation region in each 50 ms image, with a FOV of ~930x930  km, divided into 256 pixels yielding high spatial resolution. The full microburst precipitation  region is imaged by traversing the radiation belt L-shells 4 times each orbit, and gradually  precessing through all magnetic local times over the course of the mission. The X-ray  measurements will be used to infer the precipitating electron spectra and flux, which is then  further constrained by the direct measurements from the particle detector. Because the particle  detector will have enhanced energy resolution over previous missions, we will be able to detect  energy dispersion that occurs due to a combination of energy-dependent propagation effects.  This dispersion can be used to determine the generation region of the observed microbursts.  Microburst electrons are theorized to be scattered into the loss cone at off-equatorial magnetic  latitudes, where chorus waves have propagated from the equatorial source region. Previous  observations of microbursts at LEO have shown some evidence of energy dispersion, but the  limited energy resolution of the particle detectors precluded any confident dispersion  quantification that could be used to identify the microburst source location. The IMPAX detector  I-FIRE will easily resolve this energy dispersion, and the combination of these observations with  the X-ray images will allow us to directly answer the Science Questions.