EMFISIS/MAG Release Note March 12, 2023

ACE/MAG has recently adapted spectrogram codes written for RBSP/EMFISIS/MAG 
and are now making those daily spectrograms available to the public.  Credit 
for this effort goes to Dr. Matthew Argall.  While we are hesitant to use 
them in the detailed analyses of published scientific papers, they have 
proven useful in finding dynamics such as shocks and foreshock waves.

Files currently available span DOY 243 of 1997 through DOY 017 of 2023.

Daily pectrograms are made using full resolution 3 vector/sec MAG data.  
Standard FFT techniques are employed.  Short data gaps are interpolated.  
Too much missing data results in the loss of spectral information for that 
time interval.

Horizontal lines in all but the top panel denote the cyclotron frequency 
for singly charged Hydrogen (blue), Helium (peach) and Oxygen (pink).

Top to bottom, the panels are:
	Total field strength |B| (black) and the radial component BR (blue).
	Trace of the power spectral matrix (total power in the fluctuations).
	Percent polarization of the fluctuations.
	Ellipticity (positive for right-handed waves in spacecraft frame).
	Arccos(k.B) (angle between the minimum variance direction and mean B).
	Coherence of the fluctuations.

The imaginary part of the covariance matrix of the magnetic field contains 
information only about circularly polarized signals [Means, 1972]. From this, 
we can determine the components of k and transform into the wave normal system, 
with k along Z and B in the XZ-plane. In this system, the plane wave power is 
contained in the transverse plane, allowing us to separate the unpolarized and 
polarized components [Rakin and Kurtz, 1970; Fowler, et. al, 1967]. Wave 
polarization properties are then solved for in term of the components of the 
covariance matrix.

As an example of their usefulness, consider day 97 of 1998. An interplanentary 
shock is clearly evident in all panels at 17:00 UT. Right-hand polarized waves 
are clearly evident for about 10 hours prior to the shock crossing makeing this 
one of the most protracted foreshock observations in the ACE catalog. This event 
was studied by Tokar et al. [JGR, A105, 7521-7531, 2000] and the availability 
of this spectrogram would have made that study much easier.

For questions regarding data quality and availability please contact:
	Prof. Charles W. Smith, MAG Data Manager,
	Space Science Center, University of New Hampshire, Charles. Smith@unh.edu