   << Description of Akebono VLF-ELF in Science Database Format >>>

                                        Sep. 15, 1996 (Version 3.01)

The ELF which is one of subsystems of VLF instrument onboard Akebono 
is operated in either 4 channel mode (narrow mode) 
or 2 channel mode (wide mode), selectable by command.
In the narrow mode, waves in a frequency range less than 80Hz are observed.
The four channels are composed of Ch.1; Ex or Ey (selectable by command), 
Ch.2; sBx,  Ch.3; sBy and Ch.4; sBz (the characters 's' for B components
stand for search coil).
In the wide mode, the upper limit frequency is increased up to 160Hz,
but only two components, 1-electric and 1-magnetic field can be observed.

In the present science database, we provide one component of E field 
and one component of B field vector with frequency less than 
80Hz with a frequency resolution of 2.5Hz.
The file name of VLF-ELF is given as 'yymmddhh.elf'.

The file is composed of a series of blocks, each of which 
consists of 976bytes.

The first block is for the header whose format is :

 0----+----1----+----2----+----3----+----4---(...)----
  yymmddhhmmss YYMMDDHHMMSS VLF-ELF Ver.3.01 

  yymmddhhmmss: Start date and time (UT) 
                    at the 1st record of the 2nd block (ASCII)
  YYMMDDHHMMSS: End   date and time (UT) 
                    at the last record of the last block (ASCII)
  Ver.3.01    : Version Number

The blocks following the 1st block are the ELF wave data.
One block datum consists of block number (1byte) and 15 records 
(1record = 65bytes), each of which is corresponding to every 
8sec interval (1block = 1 + 65*15 = 976bytes).
Therefore each data block is constructed as follows:

    byte   |  contents    | Elapsed time(*)
 ----------------------------------------------------------------
         1 | block number | (NO)
   2 -  66 |   1st record | yymmddhhmmss + NO*120(sec)
  67 - 131 |   2st record | yymmddhhmmss + NO*120(sec) +   8sec
 132 - 196 |   3rd record | yymmddhhmmss + NO*120(sec) +  16sec
     :     |      :       |
 912 - 976 |  15th record | yymmddhhmmss + NO*120(sec) + 112sec

Each record consists E and B field intensities of 8sec average 
at 32 frequency points (from 1st to 64th byte) given by FFT processing 
and flags (65th byte) for observation status.
The 8sec average is taken for an interval from 4sec before 
to 4sec after the elapsed time. E and B intensity data and flags are 
represented by 1byte integer (I) with no sign bit as follows:

    byte   |  contents    
 ----------------------------------------------------------------
   1 -  32 |   E-field
  33 -  64 |   B-field
        lags for observation status 

The intensities are described in [dB] (0<I<254) where
0[dB]=10^-6[mV/m] for E-field and 0[dB] = 10^-6[pT] for B-field.
The unit byte is filled by I=255 for each data point where data are not 
available. 

The contents in the flag byte are as follows:

  (Upper)  8-7: Saturation percent of E-sensor *1
           6-5: Saturation percent of B-sensor *2
           4-3: Blank
             2: BDR (0: OFF 1: ON) *3
  (Lowest)   1: SMS (0: OFF 1: ON) *4

    *1 Saturation percent of E-sensor
          Saturation percent is defined by the ratio of saturated 
          data points to all data points (1280 points) in 8sec.

         8th bit  7th bit : Saturation percent of waveform in 8sec.
         ----------------------------------------------------------
            0        0    :  0 % -  5 %
            0        1    :  5 % - 10 %
            1        0    : 10 % - 20 %
            1        1    : 20 % -

    *2 Saturation percent of B-sensor
          (Same as E-sensor described above.)

    *3 BDR (Bubble Data Recorder onboard Akebono)
        When the BDR is on (both in record and reproduce mode), 
        interference at 32Hz is involved in the ELF data.

    *4 SMS (Superthermal ion mass spectrometer)
        When the SMS instrument is on, impulsive noise interferes
        with the ELF subsystem.

    !!!! Note that the interference by BDR nor SMS is *not* yet removed 
         from the VLF-ELF data !!


***********************************************************************
  To use the data for any purpose, you are requested to contact 
  the VLF PI, Iwane Kimura. Before you give a presentation 
  in meetings or publish a paper using VLF-ELF data, please 
  send a copy of the abstract or paper to PI:

   PI: Iwane Kimura,
         Dept. of Information System, Osaka Institute of Technology,
         Kitayama, Hirakata 573-01, Japan
         Phone: +81-720-66-5402
         FAX  : +81-720-66-8499
         E-mail ikimura@is.oit.ac.jp

   For further information, please contact with:

       Yoshiya Kasahara,
         Graduate School of Electronics and Communication, Kyoto University,
         Sakyo-ku, Kyoto 606-01, Japan
         Phone: +81-75-753-4983
         FAX  : +81-75-751-8201
         E-mail : kasahara@kuee.kyoto-u.ac.jp

Reference 

Kimura, I., K. Hashimoto, I. Nagano, T. Okada, M. Yamamoto, 
T. Yoshino, H. Matsumoto, M. Ejiri, and K. Hayashi,
  VLF Observations by the Akebono (EXOS--D) Satellite,
  J. Geomagn.  Geoelectr., 42, 459-478, 1990.

Hashimoto, K., I. Nagano, M. Yamamoto, T. Okada, I. Kimura,
H. Matsumoto, and H. Oki,
  EXOS-D (AKEBONO) Very Low Frequency Plasma Wave Instruments (VLF),
  IEEE Trans. Geoelectr. and Remote Sensing, in print, 1996.

-------------------------------------------------------------------------
  SAMPLE PROGRAM (Program consists of 2 parts written in C and FORTRAN)
   (for use on SUN)

---(Part 1 : C language)---
#include        <stdio.h>
#include        <stdlib.h>

FILE            *fp;
unsigned char   buf[976];

opnrelf_(file, l)
char    *file;
int     l;
{
        register unsigned char  *p = buf;
        register int            i;

        if ((fp = fopen(fs2cs(file, l), "r")) == NULL) {
                perror(file);
                exit(1);
        }
}

clsrelf_()
{
        fclose(fp);
}

gtelhd_(head, ver, stm, etm)
double  *stm;
double  *etm;
char    head[10], ver[10];
{
        char astm[15], aetm[15];
        fseek(fp, 0L, 0);

        fread(buf, sizeof(*buf), sizeof(buf), fp);

        sscanf(buf, "%s %s %s %s", astm, aetm, head, ver);
        *stm=atof(astm);
        *etm=atof(aetm);
}

int irdblke_(iblock)
long    *iblock;
{
        register unsigned char  *p = buf;
        register int            i;

        if (fread(buf, sizeof(*buf), sizeof(buf), fp) == 0)
                return(-1);

        for (i = 0; i < 976; i++) {
                *iblock++ = *p++;
        }
        return(0);
}

---(Part 2 : FORTRAN language)---
      implicit real*4(a-h,o-z),integer*4(i-n)
      character*20 file
      character*10 head, ver
      real*8       stm, etm

      narg = iargc(0)
      if(narg.le.0) then
        write(0,1000)
 1000   format('Usage: rdelf filename(yymmddhh.elf)')
        call exit(1)
      end if

      call getarg(1,file)
      call opnrelf(file)

      call gtelhd(head, ver, stm, etm)
      print*, 'head=', head, ' ', ver, stm, etm

      call readelf
      call clsrelf

      stop
      end

************************************************************************
      subroutine readelf
************************************************************************
      implicit real*4(a-h,o-z),integer*4(i-n)
      parameter (nwrd =  976)
      parameter (nblk =   90)
      common /elfdata/ iblock(nwrd,nblk), num

      num = 1
    1 continue
      irc = irdblke(iblock(1,num))

      if (irc.ne.0) goto 999

      num = num + 1
      goto 1

  999 continue

      num = num - 1
      return
      end