About MAG Data





The MAG Instrument on ACE

The Magnetic Field Experiment (MAG) consists of twin vector fluxgate magnetometers controlled by a common CPU. The sensors are mounted on booms extending 4.19 meters from the center of the spacecraft at opposite sides along the +/-Y axes of the spacecraft. The instrument returns 6 magnetic field vector measurements each second, divided between the two sensors, with onboard snapshot and FFT buffers to enhance the high-frequency resolution.

For more information about the MAG instrument, visit the MAG Home Page, at Bartol Research Institute.

June 17, 2014: MAG data from 2013/151 onwards have been re-released with new calibrations. Less than 0.2nT changes...see the release notes for details (links below...)


MAG Data Description

Time Data
All level 2 data records for all ACE instruments contain timing information in the same format. The format of the timing information is described
here.

MAG level 2 data contains one extra time data item: the spacecraft clock time (32-bit unsigned int), which counts minor frames since the start of the mission.

Interplanetary Magnetic Field Data
MAG level 2 data is organized into 27 day time periods (Bartels Rotations - roughly one solar rotation period). For each Bartels Rotation, the level 2 data contains time averages of the magnetic field data over the following time periods:

Release notes for MAG level 2 data - provided by the MAG instrument team. All users of MAG data should read these notes.
Release notes for the 1-second averages.

Instrument calibration method used to produce MAG Level 2 data - provided by the MAG instrument team.

A data value of -999.9 indicates bad or missing data for the given time period.

Magnetic field vectors are given in the RTN, GSE, and GSM coordinate systems.

For daily and Bartels rotation averages, only Bmag data is valid. For Br, Bt, Bn, Delta, Lambda, Bgse_x, Bgse_y, and Bgse_z, the fill value (-999.9) is output. Individual vector components on timescales longer than 1 hour are considered to be potentially misleading and have questionable value at 1 AU. For this reason, they are not computed. Values of <|B|> may be more useful, but the user is cautioned to examine the dBrms value for these intervals as large values of dBrms may indicate significant changes in the magnetic field during the averaging interval.

Data error is less than 0.1 nT (where nT is nano-Tesla).

The averaged Mag data, which are all float32, are:

	Br	- The r component of the magnetic field vector in the RTN 
		  coordinate system in nT 
	Bt	- The t component of the magnetic field vector in the RTN 
		  coordinate system in nT
	Bn	- The n component of the magnetic field vector in the RTN 
		  coordinate system in nT
	Bmag	- The <|B|> magnetic field magnitude in nT 
	Delta	- The angle in degrees with 0 at Br/Bt plane + toward Bn 
		  (-90 to +90 degrees), i.e. the RTN latitude
	Lambda  - The angle in degrees with 0 at Br and + toward Bt 
		  (0 to 360 degrees), i.e. the RTN longitude
	Bgse_x	- The x component of the magnetic field vector in the GSE 
		  coordinate system in nT 
	Bgse_y	- The y component of the magnetic field vector in the GSE 
		  coordinate system in nT 
	Bgse_z	- The z component of the magnetic field vector in the GSE 
		  coordinate system in nT 

	Bgsm_x	- The x component of the magnetic field vector in the GSM
		  coordinate system in nT 
	Bgsm_y	- The y component of the magnetic field vector in the GSM 
		  coordinate system in nT 
	Bgsm_z	- The z component of the magnetic field vector in the GSM 
		  coordinate system in nT 

	dBrms	- RMS values of underlying high-resolution measurements in nT.
		  This is the rms variation of the vector over the time 
		  interval, calculated as follows:
			1) calculate the average magnitude of each of the three 
			   components of the vector, over the time interval.
			2) for each component, average the square of the
			   difference between the measurement and the average
		        3) add the averages from the three components
			4) take the square-root of the result.
		 i.e.
  
		       dBrms = sqrt(sum_i <( B_i -  )^2>)

		 For the 16-second averages, dBrms is calculated using the
		 the highest resolution data (3 vectors/second, from one of the
		 two sensors). For the 4-minute and hourly averages, the 16-second
		 averages are used as input to the calculations.

	sigma_B	- The variance of |B| over the time interval, in nT.

		  i.e. sigma_B = sqrt(<(|B|-<|B|>)^2>).

		  If sigma_B/<|B|> is small, there was little variation 
	          during the time period.
		  If sigma_B/<|B|> is large , there was much variation 
	          during the time period.

		  sigma_B is only calculated for the 4-minute and hourly
		  averages. It is calculated using the 16-second averages
		  as input.

Quality information about the Mag data, float32 except as noted:

	fraction_good	
		 - Fraction of the timing period for which there was data
		   available for the time period.

	N_vectors- (in int32) Number of 16 second vectors that were included
		    in the average.

The data quality for the period (int32); data of type 2 have been excised,
i.e. replaced with fill data (-999.9):
	
	Quality	= 0 Normal data
                = 1 Spacecraft Maneuver & subsequent high-nutation period (~4 hr)
		= 2 Bad data/missing data

ACE spacecraft position, in GSE and GSM coordinate systems (float32):

	pos_gse_x - The x-component of the spacecraft position,
	            in GSE, in km.
	pos_gse_y - The y-component of the spacecraft position,
	            in GSE, in km.
	pos_gse_z - The z-component of the spacecraft position,
	            in GSE, in km.

	pos_gsm_x - The x-component of the spacecraft position,
	            in GSM, in km.
	pos_gsm_y - The y-component of the spacecraft position,
	            in GSM, in km.
	pos_gsm_z - The z-component of the spacecraft position,
	            in GSM, in km.

Download MAG level 2 data


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Last Updated: 7 November, 2007

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