Solar Radio

Introduction

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 the WDC every day.

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 Bureau of Meteorology (BOM) and USAF.

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)
• mon is the first three letters of the month name (eg, JAN, DEC)
• yy is the last two digits of the year (eg, 00 for 2000)
• 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.

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 following headings.

Know your equipment

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 Space Weather Forecasting Centre (ASWFC).

Know your environment

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.

Know your source

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:

• 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.
• 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.

Radio Solar Telescope Network (RSTN) 1-sec Solar Radio Data (SRD) files

Some general information:

File names are of the form:

LYYMMDD.SRD

For example: L080204.SRD

Each file contains 1-second records of 8 solar radio flux measurements. Each line contains the following fields (blank or zero fields indicate missing data).

• UT time - HHMMSS
• 245 MHz flux value
• 410 MHz flux value
• 610 MHz flux value
• 1415 MHz flux value
• 2695 MHz flux value
• 4975 MHz flux value
• 8800 MHz flux value
• 15400 MHz flux value

Exerpts from an example file are shown below.

000000 2101 4101 6601 1162 1212 1742 2732 4902 000001 2101 4101 6601 1162 1212 1742 2732 4902 000002 2101 4101 6601 1162 1212 1742 2732 4902 000003 2101 4101 6601 1162 1212 1742 2732 4902 000004 2201 4101 6601 1152 1212 1742 2732 4902 000005 2201 4101 6601 1152 1212 1732 2722 4892 000006 2201 4101 6601 1162 1212 1732 2722 4872 000007 2201 4101 6501 1162 1212 1742 2732 4902 000008 2201 4101 6601 1162 1212 1742 2732 4922 000009 2201 4101 6601 1162 1212 1742 2732 4902 ... 235953 1801 3801 6301 1172 1232 1772 2682 4892 235954 1801 3801 6401 1172 1232 1772 2682 4872 235955 1701 3801 6401 1172 1232 1772 2662 4872 235956 1701 3801 6401 1162 1222 1742 2652 4842 235957 1701 3801 6401 1162 1222 1742 2652 4832 235958 1701 3801 6401 1162 1222 1742 2652 4842 235959 1801 3801 6301 1172 1222 1742 2652 4842

The four digit number abcp is the flux value a.bc*10^p. for example a value of 7453 should be read as 7.45*10^3 or 7450 SFU. The units are solar flux units (SFU) where 1 SFU is 10000 Jansky or in SI units 1 SFU = 10^{-22} Watts/meter^2/Hz.

The maximum solar radio flux observable on each frequency varies with frequency:

245 MHz      500,000 solar flux units (sfu)
410 MHz      500,000 sfu
610 MHz      500,000 sfu
1415 MHz      100,000 sfu
2695 MHz      50,000 sfu
4995 MHz      50,000 sfu
8800 MHz      50,000 sfu
15400 MHz      50,000 sfu