Geotail/EPIC Channel Descriptions

A channel is defined as a unique combination of particle species, energy range, and sensor source. For EPIC, there are also some channels delineated by time-of-flight (TOF) instead of energy. The particle species for a given channel may include a specific charge state. All told, there are 789 different particle channels in EPIC data.

Click here to see a list of all 789 channels: EPIC Channel Table

Channel Naming Conventions

Each channel name begins with a short mnemonic label, such as HE1 for the first ICS Helium channel. Following the mnemonic is a postfix which indicates the detector head or heads utilized by the channel. A sample postfix would be _H1OF6, which is read as head 1 of 6 and indicates that the channel records information about head 1 out of a possible 6. You can also safely assume that there are 5 more channels for the remaining heads with the same species and energy settings. Note that in cases where several heads are combined into one channel that you can get postfixes such as _H1OF3, where the three total heads are the three logical clusters of heads (each cluster consisting of two heads).

There is nothing in the name to indicate wether the channel comes from ICS or STICS.

Angular and Timing Resolution

Geotail is a spinning spacecraft, and for EPIC, the spin is divided into 16 sectors of 22.5 degress each. The data for some channels is available for every sector of every spin, but doing this for every channel would have required too much communications bandwidth. Therefore, some channels have only 8 sectored data, others only 4 setored data, and some are spin averages. Other channels maintain all 16 sectors, but those measurements are averages over several spins. Common options for numbers of spins summed are 1, 2, 16, or 32. (A Geotail science record on the spacecraft is 32 spins, roughly 96 seconds.)

A chart of all the channels is provided (click here to see it), and for each channel, the number of sectors summed and the number of spins summed is indicated.

ICS Channels

There are four kinds of ICS channels:
  1. Energy Channels - all particles hitting the solid state detector over a range of energies

    Example channel names: E2_H1OF2   E2_H2OF2  

    There are 16 different energy windows, spanning the energy range of 45.0 keV to 3005.4 keV. Each head uses the same energy windows. Most channels report all 16 sectors, but each sector is accumulated for 16 spins.

    The energy channels are not restricted to any species.

    There are two energy channels (E_B2_H1OF1 and E_B5_H1OF1) which are taken every secotr, every spin, and are also a combination of both detector heads.

  2. TOF channels - all particles registering in the TOF system within a list of transit time ranges

    Example channel names: T2_H1OF2   T2_H2OF2  

  3. species channels - particle types ranging from hydrogen to iron, with several energy ranges for each species type

    Example channel names, proton channels: P1_H1OF2   P2_H2OF2  

    Example channel names, heavy ion channels: H5_H1OF2   H6_H2OF2  

  4. electron channels - ICS also includes an electron detector with just two channels

    The first electron channel, ED1_H1OF1 measures electrons above 39 keV, and the second ED2_H1OF1 measures electrons above 110 keV. Note that the postfix indicating the head has been kept for consistency, even though the electron detector only has one head.

    Since both electron channels have only a single energy bound, they are reported in units of integrated intensity.

STICS Channels

STICS is an electrostatic analyzer, and it repeatedly steps through different plate voltages, letting in ions of specific energy per charge at each step. The steps are referred to as DV steps. The DV step number is recorded in the channel name. Not every DV step was used, and so the DV step indices present are spaced as in the sequence 2, 6, 10, 14, 18, 22, 26, 30.

The width of the energy channels for STICS are narrow, such that the boundaries of adjacent channels do not touch. This is illustrated in the following figure:

There are many different species in the STICS channels: protons, helium (singly and doubly charged), plus many different charge states for carbon, nitrogen, oxygen, and iron. The hydrogen ions channels are collected every sector of every spin, and each STICS sensor head is separately accounted for. But for other ions, some combining of heads, sectors or spins is used. For example, the SMR2 channels (for doubly charged oxygen), provide information from 3 clusters of 2 heads (rather than from each of the 6 heads individually), and provide 8 sectors per spin, with data from every spin (no spins are summed).

There are two sets of channels in STICS data which are different from all the other types of channels - these have the prefix label HR10 and HR11, and are for singly charged oxygen, and the front singles rates (FSR) respectivle. The FSR is just the counts per second registering in the front foil of the TOF telescope, and is an indicator of the overall ion environment that STICS is seeing at any given time. The HR10 channels provide data for every sector of every spin for 8 spins, and then no data at all during the next 8 spins. During this second set of 8 spins, this is the only time data from the HR11 channels are taken. Thus the HR10 and HR11 channels are interleaved so that you never get data from both channels during the same set of 8 spins. Note that the FSR data is mostly made available for diagnostic purposes.

Sample Channel Names and Meanings

The following table provides a sample of some of the channel names and ther explanations.
E8_H1OF2 2 1 1 16 the eighth energy window for ICS energy channels, ???? head, data for 16 sectored directions, but summed over 16 consecutive spins
E8_H2OF2 2 1 1 16 same as previous entry, but for the ??? head
E8_H2OF2


back to Space Department home page
back to MIDL home page

Maintainer:

Jon D. Vandegriff
Last modified: Wed Aug 06 12:07:17 Eastern Daylight Time 2003