The interaction between Earth's magnetic field and Solar particles is a complex and mysterious field of science. The storm events involve high electric currents in the ionosphere and vast amounts electric power affecting to great many things. One of the ways to observe what is happening up there, is to detect the effects of these phenomena to non ionizing long wave electromagnetic radiation - radio waves. Earth's magnetosphere it self is a source of electromagnetic RF energy at LF- and VLF frequencies, but since these emissions are not easy to detect from other emissions specially in rural areas, a good alternative is to monitor radio signals sent by man. Ionosphere on high latitudes is affected during geomagnetic disturbances, which can be observed on radio signals that pass through, or become reflected from the E layer of the ionosphere. Visible Auroras, which are observed at higher altitudes and require less less energy to appear, as well as Radio Auroras, requiring extremely dense ionization around 100 km to occur, are just some of the phenomena related to geomagnetic disturbances. The origin of these disturbances are the enhancements of density, energy, quality and speed in Solar particle emissions, including the polarity of the interplanetary magnetic field and coupling of the particles to Earth's magnetosphere.
Types of radio Auroras
On VHF frequencies (30...300 MHz) the most notable type of Radio Aurora is the Auroral back scatter of radio signals from pillars and arcs of collisional ionization in the E layer concentrated by ionic-acoustic waves at about 100 km heights. These reflecting irreqularites in the ionosphere are only of meter class in size. This may sound odd as the sky might simultainously be filled up with curtains of visual auroras with peak heights up to several hundred kilometers.
The propagation geometry is strictly (magnetic) field aligned (-10 dB/°). Such signals are detected up to UHF, but are more obvious on lower VHF frequencies. Exotic Auroral E propagation, occurring perhaps by the 10 keV electrons entering the cleft and becoming deposited at the midnight precipitation zone (Thule type Auroral E), sometimes occurs a few hours after night Radio Aurora, but with signal strengths that are sufficient only to be detected reliably on VHF low band (approximately 30...70 MHz).
Auroral back scatter signals are caused more directly from the Solar particle emissions captured by Earth's magnetic field and are being received often around 1600 local time. These afternoon Radio Auroras have plenty of Doppler-spread dispersing the signal's spectrum. This effect is caused by the vast mass of individual gyrating electrons reflecting the radio wave. There also can be observed Doppler shift or in fact several of them caused by the overall motion of the ionized reflecting areas in relation to the location of transmitter and receiver due to the electric field. During disturbed conditions two peak spectra can be observed from differences caused by diverse electric field conditions at differenct altitudes. The typical Doppler phase velocities range from -1000 to +1000 m/s.
A minima called the Harang discontinuity occurs around 2100 local time.
Another maxima is at midnight hours, with less Doppler hiss on the signals and slightly less strength. The particles are more of deposits from the magnetotail, rather than being related directly to Solar emissions. High local geomagnetic indices (K=9) and Radio Aurora are rarely observed during the late morning hours. Basically the same Radio Auroras are observed also on the HF part (3...30 MHz) of radio spectrum, but since other ionospheric propagation modes (F, Es) are often dominant there and noise level is higher, the interpretation of propagation mode(s) becomes difficult. A suitable set-up on VHF does not have any other than ionospheric propagation. Of these, the F layer reflections do not occur above 60 MHz and Es appears sporadically in the summer months and reducing as frequency increases, leaving in practice only Meteor Scatter and Radio Aurora as primary propagation modes.
You may study more on the behavior and physics of the Auroras from
Space Physics Textbook by the University of Oulu.
Radio observations in the Aurora section
Ilkka Yrjölä has observed Radio Aurora (24 h/day) on 87 MHz
since August 1993 (on various frequencies around 89 MHz since April 1998)
and on 144 MHz since January 1995, using automated set-up of PC, two (since
1998 three) receivers and squelch detection. This effort is a by-product
of Global-MS-Net searching for meteor outbursts and monitoring meteor rates
by receiving Meteor Scatter signals, which are sometimes disrupted
by Radio Aurora. Auroral data from this effort has been regularly included
in the Aurora Sections reports in the form of bar charts.
SK4MPI Radio Beacon
VHF back scatter from Aurora.
Receiving the Swedish Radio Aurora beacon SK4MPI on 144.412 MHz is one of the most sensitive and reliable ways for an amateur to detect Radio Aurora in Scandinavia. This beacon can usually be heard when the K index rises to 4 or higher. Observations on 50 MHz with large high gain antennas have revealed weak Auroral signals being received at high latitudes whenever the geomagnetic field is even slightly unsettled (K 1 or 2)
SK4MPI beacon located in Borlänge, Sweden, is dedicated to aid detection and study of Radio Aurora conditions on VHF in northern Europe. The beacon was originally initiated in the early 1970's by Max-Planck-Institute fur Aeronomie for scientific Aurora research and is now maintained by Amateur Radio operator SM4HFI. SK4MPI is the most powerful continuously transmitting Amateur Radio beacon in Europe on the 144 MHz band.
It has ERP of 1.5 kW to two broad beams towards N/W and
N/E. The benefit of using this station to detect Aurora, is that there
are no other transmissions on the same frequency.
The beacon is not received in Kuusankoski, Finland, if there is no Aurora, except for some short and random meteor reflections. As Auroral conditions begin, back scattered signal comes up from noise. With moderate receiving antennas the signal may peak up to 20 dB above noise level. The beacons keying cycle consists of a 1 minute long message with A1A Morse coded call sing & locator and the rest 45 seconds is carrier signal (A0).
Green curve indicates strength (large antenna) and bar the detection (smaller antenna) exceeding expressed threshold of back scatter signal from Sweden. Red bar shows detection of signals from Germany on 87 MHz and cyan the detection of Dutch TV carrier on 62.2 MHz. Notice the later appearance of long distance signals from west compared to back scatter from north.
To receive the signal, a narrow band receiver for 144.412 MHz is needed along with a Yagi antenna with 4, or more elements. A 12 element Yagi and a narrow band FM receiver with -125 dBm threshold, has been used to good effect on this project. A location with low RF noise and free takeoff towards north is of course necessity. Converting F/S information from the receiver with an A/D converter and feeding it to a computer that logs the data, is one way to gather information on Radio Aurora. Sampling can be relatively slow and F/S signal can be heavily integrated.
Below is a sample of Radio Aurora report:
Radio Aurora report for 04-98, 144 MHz back scatter. E > 0.3 uV/m (-10.5 dBuV/m) over 30 sec/10 min. date start end dur. 10-04-98 2010...2040 UT 0.50 h 10-04-98 2110...2150 UT 0.67 h 11-04-98 2120...2130 UT 0.17 h 11-04-98 2140...2150 UT 0.17 h 11-04-98 2200...2220 UT 0.33 h 11-04-98 2230...2400 UT 1.50 h 12-04-98 0000...0100 UT 1.00 h 12-04-98 0210...0230 UT 0.33 h 12-04-98 0320...0330 UT 0.17 h 17-04-98 1350...1510 UT 1.33 h 20-04-98 1350...1430 UT 0.67 h 23-04-98 2340...2400 UT 0.33 h 24-04-98 0000...0030 UT 0.50 h 24-04-98 0100...0250 UT 1.83 h 24-04-98 0530...0550 UT 0.33 h 24-04-98 1040...1050 UT 0.17 h 24-04-98 1250...1300 UT 0.17 h 24-04-98 1330...1420 UT 0.83 h 24-04-98 2250...2300 UT 0.17 h 24-04-98 2310...2400 UT 0.83 h 25-04-98 0210...0230 UT 0.33 h 25-04-98 0240...0300 UT 0.33 h 25-04-98 0310...0330 UT 0.33 h 25-04-98 0350...0400 UT 0.17 h 25-04-98 1330...1340 UT 0.17 h 25-04-98 1550...1610 UT 0.33 h 25-04-98 1630...1650 UT 0.33 h 26-04-98 0100...0140 UT 0.67 h 26-04-98 1320...1550 UT 2.50 h 26-04-98 1610...1700 UT 0.83 h 26-04-98 2200...2220 UT 0.33 h Radio Aurora & ES report for 04-98, 62 MHz, path: Holland-Finland. E > 0.2 uV/m (-14 dBuV/m) over 1.2 min/10 min. date start end dur Prop.mode 10-04-98 2020...2030 UT 0.17 h Aurora -------------------------------------------------------------------
Examples of Aurora signals
Audio spectrogram of SK4MPI, a 144.412 MHz Amateur Radio beacon for aurora taken 14.11.1997 by OH5IY. The pure narrow carrier is smeared by Doppler-spread effect in to noise several hundreds of herzes wide.
Spectrum of two TV carriers on 49.76 MHz. The lower frequency means there is less Doppler-spread. The slow wobbling of the carrier originates from the transmitters. The typical fast flutter is from Aurora's Doppler-shift.
Radio Auroras, Charlie Newton, 1st Ed., 1991, RSGB, ISBN: 1-872309-03-8.
Thanks for V.K Lehtoranta for sharing his views and sending related material.