MESSENGER Mercury Energetic Event Table (MEAP-3)

MEAP3 – Energetic Electron Event Table Derived Data Record

PDS Bundle: urn:nasa:pds:izenberg_pdart14_meapDOI: 10.17189/1518648

PDS Collection: urn:nasa:pds:izenberg_pdart14_meap:data_eetable

For more information about MEAP3 products, see Lawrence et al., 2015, “Comprehensive survey of energetic electron events in Mercury’s magnetosphere with data from the MESSENGER Gamma-Ray and Neutron Spectrometer”, doi:10.1002/2014JA020792.

For more information about MEAP products, see the Messenger Advanced Product Specification.

The Neutron Spectrometer (NS) Energetic Electron Event Table Derived Data Record (MEAP3) tables contain information about energetic electron (EE) events detected by the NS throughout Messenger’s orbit of Mercury. The tables include event characteristics, timings, intensities, and other information derived from the NS calibrated data. Each event is divided into records which cover a 20-second accumulation of counts.

The NS was designed to observe the neutrons emitted from Mercury’s surface in the thermal, epithermal, and fast energy ranges, from ~0.01 eV to 7 MeV, that are produced by nuclear reactions by the cosmic ray background.

The NS indirectly detected EE events via bremsstrahlung photons that were emitted when instrument and spacecraft materials electrons with energies of tens to hundreds of keV. The NS had an order-of-magnitude larger sensitivity for EE events than the Messenger Energetic Particle Spectrometer (EPS). At the time that the Messenger data archive was being planned, it was not anticipated that the NS would provide such a robust measure of EE events. Now that its capability has been demonstrated, these data provide a valuable resource for many kinds of studies of Mercury’s magnetosphere.

The duration of EE events ranged from tens of seconds to almost 20 minutes. EE events were classified as bursty or smooth, such that bursty events showed large count rate variability within an event, and smooth events showed small count rate variability. Almost all EE events were detected within Mercury’s magnetosphere on closed field lines. The occurrences of EE events are stochastic in nature, but are located in well-defined regions with clear boundaries that persist in time, and form “quasi-permanent structures”. Bursty events occur closer to dawn and at higher latitudes compared to smooth events, which are seen near noon-to-dusk local times at lower latitudes. A subset of EE events showed strong periodicities that ranged from hundreds of seconds to tens of milliseconds.

MEAP3 parameter definitions:

Parameter	Description
Event number	Unique identifier for each EE event.
Event length	Duration of the event in number of 20-second accumulations.
Day of year	Day of year on which the 20-second accumulation occurs.
Month	Month in which the 20-second accumulation occurs.
Day	Day in month in which the 20-second accumulation occurs.
Year	Year in which the 20-second accumulation occurs.
Hour	Hour within the day in which the 20-second accumulation occurs.
Minute	Minute within the hour in which the 20-second accumulation occurs.
Second	Second within the minute in which the 20-second accumulation occurs.
MET	Mission elapsed time, and is the mission tag time in seconds of the start of the associated accumulated period. Times prior to 9 January 2013 are in the first MET partition; times on and after 9 January 2013 are in the second MET partition.
Orbit number	Orbit number is defined as starting at apoherm and is calculated using the MET value and the appropriate SPICE kernels. Orbit numbering does not start until Messenger performs the Mercury orbit insertion. Until that time the value for orbit number is 0.
Altitude	Spacecraft altitude above the subsatellite point on the target in units of km.
Latitude	Target-centric latitude of the spacecraft subsatellite point in degrees.
Longitude	Target-centric longitude of the spacecraft subsatellite point in degrees.
Local time	Local time of the spacecraft subsatellite point in hours from 0 to 24.
Beta angle	Angle of the normal of the spacecraft subsatellite point in hours from 0 to 24.
Sun distance	Distance of the spacecraft to the Sun in units of km.
Periapsis latitude*	Target-centric latitude of the spacecraft when it is at the periapsis (lowest) altitude for the given orbit number.
Event length, minutes	Length of the EE event in minutes. The value is repeated for ‘Event Length’ rows in the file.
Signal-to-noise	Event signal-to-noise, and is the measure of the size of the even within each 20-second accumulation.
BP total	Total counts within the Borated Plastic sensor 64-channel, 20-second spectral accumulation.
BP low	Total counts within the lowest 12 channels of the Borated Plastic sensor, 20-second spectral accumulation.

* In a small number of cases, the periapsis latitude has values of 0. This is due to an artifact in the algorithm that determines this value. Since this value is determined by the spacecraft orbit, approximate periapsis latitude values can be determined by interpolating from the values of events located nearby in time.

There are three MEAP3 products:

ELE_EVT_12HR_ORBIT_2011-2012.TAB

Data from the 12-hour orbit, taken during 2011 and 2012.

ELE_EVT_8HR_ORBIT_2012-2013.TAB

Data taken from the 8-hour orbit, taken during 2012 and 2013. The data from 2011 to 2013 encompasses the entire dataset reported in Lawrence et al. (2015, doi:10.1002/2014JA020792) and contains 2711 events.

ELE_EVT_8HR_ORBIT_2014-2015.TAB

Data taken from the 8-hour orbit in the years 2014 and 2015. This contains an additional 1400 events. The total number of EE events for the mission is therefore 2711 + 1400 = 4111.

MEAP3 PDS4 Logical Identifiers:

urn:nasa:pds:izenberg_pdart14_meap:data_eetable:ele_evt_12hr_orbit_2011-2012

urn:nasa:pds:izenberg_pdart14_meap:data_eetable:ele_evt_8hr_orbit_2012-2013

urn:nasa:pds:izenberg_pdart14_meap:data_eetable:ele_evt_8hr_orbit_2014-2015