Introduction to Solar Radio Data

The Learmonth Solar Radio Spectrograph observes the radio emmission of the Sun from 25MHz to 180 Mhz. Solar radio spectrograph display radio bursts or "sweep" events. These are classified into particular types. A "Type 2" spectral burst is believed to be due to plasma emmission that occurs following the passage of a shock wave through the corona, usually associated with a solar flare. This information can be used to try and predict the arrival time of the shock at the Earth, and the possible onset of geomagnetic storm activity.

Learmonth solar radio data is archived automatically into WDC everyday.

1. The USAF Radio Solar Telescope Network (RSTN) Observatories

The US Air Force operates four solar radio observatories at various locations around the world. These are collectively known as the Radio Solar Telescope Network or RSTN. Each observatory monitors solar radio emissions on 8 discrete fixed frequencies (245, 410, 610, 1415, 2695, 4995, 8800 and 15400 MHz) as well as low frequency spectral emissions in the VHF band.

This document is only concerned with the 8 discrete frequencies.

The four RSTN observatories are: Sagamore Hill (K7OL), Palehua (PHFF), Learmonth (APLM), San Vito (LISS).

The Learmonth Solar Observatory is jointly operated by IPS and USAF.

2. RSTN Data Archive

RSTN operations started in the mid 1970's and each of the 8 radio telescopes was connected to one or more chart recorders. In the late 1970's a group of programmers at Palehua Solar Observatory wrote a suite of programs for the observatory HP1000 computer to digitise, analyse and archive data from the 8 discrete frequencies. This data was stored in a binary format on standard 9 track magnetic tape.

Early in the 21st century, the USAF decided that the HP1000 was to be replaced for RSTN operations by a PC based system. This was termed the RSTN Rehost, and the code was written by USAF programmers at Sagamore Hill Air Force Base in Ogden, Utah. This system was called the Fixed Frequency Analysis Program.

"APL" Extension files - The RSTN3 Data Archive Format (an ASCII format)

This format contains ASCII records, with one record per second. Each record contains data from all eight discrete frequencies. Each record is terminated by a carriage return followed by a line feed . Each record, including terminating characters is 68 bytes long. A full summer day's file may reach 3.5 Mega-bytes in size.

Data records are written to local day files with the naming convention:

ddmonyy.APL

where:
dd is the local day of the month (eg, 01, 02, 03, ... , 30, 31) - 2 characters
mon is the first three letters of the month name (eg. JAN, DEC) - 3 chars
yy is the last two digits of the year (eg 00 for 2000) - 2 characters
OBS is the first three characters of the Observatory ID (eg. APL, PHF )

Note that a UT (Universal Time) boundary may be crossed in a local day file. However, only one file is associated with each observatory local day.

Each record has the format:

OBSCYYYYMMDDHHmmSSFFF245FFF410FFF610FF1415FF2695FF4995FF8800F15400CRLF

where each character above represents a single byte, and where:

OBSC is the four character observatory code (APLM, PHFF, K7OL or LISS)
YYYY is the year (eg. 2000)
MM is the month number (eg 01 for January, 12 for December)
DD is the day of the month (eg 01 to 31)
HH is the hour (UT) the data was collected
mm is the minute (UT) the data was collected
SS is the second (UT) the data was collected
FFF245 is the solar radio flux observed on 245 MHz (rounded to the nearest integer)
FFF410 is the solar radio flux observed on 410 MHz (nearest integer)
FFF610 is the solar radio flux observed on 610 MHz (nearest integer)
FF1415 is the solar radio flux observed on 1415 MHz (nearest integer)
FF2695 is the solar radio flux observed on 2695 MHz (nearest integer)
FF4995 is the solar radio flux observed on 4995 MHz (nearest integer)
FF8800 is the solar radio flux observed on 8800 MHz (nearest integer)
F15400 is the solar radio flux observed on 15400 MHz (nearest integer)
CRLF are the two termination characters as explained above

Notes:

Six characters for each frequency allow flux values up to 999,999 SFU to be recorded. As none of the RSTN radio telescopes records above 500,000 SFU, there is ample headroom.

The flux values are padded with leading blanks If the flux happens to be negative (a physical impossibility, but one which can occur when the antennas are off the Sun due to calibration tolerances), or the frequency is not working or is in calibration mode, then the flux value for that frequency is set to six blanks. Blank entries thus indicate no useful data.

3. Data Interpretation

Routine radiotelescope calibrations are carried out twice a day, one in the morning, shortly after the radiotelescopes begin recording data, and another around noon-time. Note that during these times, the traces will usually disappear from the screen. The sequence of calibration is to take first a cold sky reading, with the gain of part of the radiotelescope different from the normal "track" values. Then, with the same gain values, a reading of a standard noise source is made. Finally, with the gain settings now returned to normal patrol values, a reading of the cold sky is made again. In the noon calibration, these three readings are followed by a drift scan to more accurately determine a value for the background solar fluxes around midday.

Not all the deflections of the trace will be due to solar activity. Unfortunately, from time to time radio frequency interference will be recorded. This will vary from frequency to frequency and from site to site. Some sites are more prone to RFI than are other sites. Some frequencies at some sites are also more susceptible to interference than other frequencies. Interference is generally from man-made transmitters, both on the ground and in space, although both man-made and natural electrical discharges can also cause RFI, particularly on the lower frequencies.

There are three basic ways in which it may be possible to distinguish true solar emissions from RFI. These essentially come under the headings:
(1) Know your equipment
(2) Know your environment
(3) Know your source.

(1) A knowledge of the equipment, its centre frequency, bandwidth, beamwidth and dynamic range can be most useful. This has been published in a paper in "The Australian Physicist", among other sources, and can be made available through application to the Australian IPS Radio and Space Services .

(2) Knowing the location of the site in question and a knowledge of the local transmitters can help find or confirm RFI sources. Such information is generally available from the communications agency of the host country

(3) Knowing the types and morphology of emission from the Sun can many times identify non-solar sources very rapidly. There are two useful rules of thumb here:

(a) solar bursts generally show a fast rise to maximum (although not an abrupt increase), followed by a slower decay. The profile of the rise and decay tend to be of parabolic form.

(b) although very rapid solar "bursting" can occur on the lower frequencies, this is not so of the higher frequencies (due to the 'inertia' of the plasma at the greater densities from which these emissions come). Any spiky burst on 2695 MHz or above is thus very likely to be non-solar.

A further and sometimes more immediately practical technique for identifying non-solar emissions is to compare contemporaneous data from two or more sites (if possible). If only one site shows the emission, it is probably not solar. If the same emission profiles are recorded by two or more RSTN sites, the emission is most probably solar in origin.

Lastly we should note that the data in each file will cover a local day (not a UT day). It also may start before or after local sunrise, and it may end before or after local sunset. As a general rule, each radiotelescope should be tracking the Sun between an elevation angle of 3 degrees in the east to 3 degrees in the west, although some sites may vary this rule due to local obstructions. In any case, it would be wise for RSTN data users to be aware of local sunrise and sunset times for the site that collected the data with which they are working.

4. The Three RSTN Data Formats

In summary, data from RSTN Observatories may be found in 3 formats: the original HP1000 format (*.LSR, which we do not discuss here), the Old FFAP Format, and the New FFAP format (such as *.APL). The New FFAP data format is used by the current system, the RSTN Rehost. The viewer described in this document will display data in the New FFAP data format which we are now referring to as RSTN3 data:

Designation	Format	Data Dates	IPS Holds
RSTN1	HP1000 binary format (YYYY_DDD.LSR)	1980-1999	1991-2001
RSTN2	Old FFAP data format (DDMMMYY.APL for Learmonth)	1997-2000	2002-2007
RSTN3	New FFAP/Rehost format (DDMMMYY.APL for Learmonth)	1999 - present

The other two formats are IPS proprietary / USAF ... the old 1-minute "rdata" and the newer 1-s "*.SRD" that now arrives in the ASFC and IPS.

The *.SRD data files from Learmonth contain exactly the same information as the RSTN3 (*.APL) files albeit in a different ASCII format. The *.SRD files are continuing to flow into the ASFC and IPS. RIMS (Radio Interference Measurement System/Radio Interference Monitoring Sets) viewer uses these files. The *.APL files arrive once every ~6 months on CD.

5. RADIO SOLAR TELESCOPE NETWORK (RSTN) 1 Sec Solar Radio Data (SRD) files

Location: Learmonth Solar Observatory
Organization: Joint IPS/USAF
Geographic Latitude: -22.25
Geographic Longitude: 114.08
Cadence: 1 Second
Values:peakflux values
Frequencies: 245, 410, 610, 1415,2695, 4975, 8800 and 15400 MHz
Period: Local day, sunrise to sunset eg 22-10UT.

FILENAMES: LYYMMDD.SRD

Example: L080204.SRD

FILE CONTENTS: 1 second records of 8 solar radio flux measurements.

Example: file start and end. blanks or zeros appear to be "no data"

UT Eight Discrete Solar radio flux values
HHMMSS 245 410 610 1415 2695 4975 8800 15400
_______________________________________________
000000 7000 2001 2101 5701 6401 1152 2022 4952
000001 7000 2001 2101 5701 6401 1152 2032 4952
000002 7000 2001 2101 5801 6601 1152 2032 4962
000003 7000 2001 2101 5801 6601 1132 2022 4962
000004 7000 2001 2101 5701 6401 1132 2012 4952
000005 7000 2001 2101 5601 6401 1122 1992 4892
000006 7000 2001 2201 5501 6401 1122 2012 4862
000007 7000 2001 2201 5601 6401 1122 2012 4852
000008 7000 2001 2201 5701 6401 1132 2022 4882
000009 7000 2001 2201 5701 6401 1132 2022 4882
.
.
.
.
.
105150
105151
105152
105153
105154
105155
105156
105157
105158
105159


BTW, *.LSR data file has different file name format with *.SRD file, but both of them have the same file content format as shown here.

The HP data format is as follows:
HHMMSS abcp abcp abcp abcp abcp abcp abcp abcpCRLF
where:
HH - Hours
MM - Minutes
SS - Seconds

The abcp fields are the flux values for each frequency. In order these are: 245 MHz, 410 MHz, 610 MHz, 1415 MHz, 2695 MHz, 4995 MHz 8800 MHz and 15400 MHz. Each abcp field should be read as a.bc*10^P. For example a value of 4073 should be read as 4.07*10^3 or 4070.
CR is the ASCII carriage return character and LF is the ASCII line feed character (DOS and windows standard end of line).

6. RADIO SOLAR TELESCOPE NETWORK (RSTN) 1 Minute Solar Radio Data (SRD) files

Learmonth Discrete Frequency Radio Flux data

Location: Learmonth Solar Observatory
Organization: Joint IPS/USAF
Geographic Latitude: -22.25
Geographic Longitude: 114.08
Cadence: 1 Minute
Values:peakand mean flux values
Frequencies: 245, 410, 610, 1415,2695, 4975, 8800 and 15400 MHz
Period: Local day, sunrise to sunset eg 22-10UT.
No data indicator: 99999
Data Gaps: Yes. hour(s) can be missing.
Data Available: 2003 (july-Dec) 2004(Jan-Nov) Jan 2005 onwards

Data Files:

rdata.DD.MM.YY

DD day of month
MM month
YY year

Data File Format Description:

Data Line is as follows:

Radio Solar Flux Unit Measurements

Hour Minute

245Mhz

410Mhz

610Mhz

1415Mhz

2695Mhz

4975Mhz

8800Mhz

15400Mhz

|---------|

|---------|

|---------|

|---------|

|---------|

|---------|

|---------|

|---------|

HH MM

peak

mean

peak

mean

peak

mean

peak

mean

peak

mean

peak

mean

peak

mean

peak

mean

00 00

10.00

9.90

27.00

26.73

34.00

32.98

49.00

48.51

76.00

74.48

120.00

117.60

180.00

178.20

460.00

455.40

Expressed as C format statement:
Time field Data field 8 pairs of values
"%02d %02d " "%9.2f %9.2f"