This region of the spectrum includes radio frequencies in the MF ranges from 300 kHz to 3 MHz (Medium Frequency) and HF 3 MHz to 30 MHz (High Frequency), typically used by commercial broadcasting services and by radio amateurs.
As is known, due to the atmospheric transparency, are possible radioastronomical observations from ground at minimum frequencies of the order of 10 MHz (value widely variable according to the solar activity, geographic region, the season, day and night).
In these bands are particularly intense artificial disturbances typical of urban and industrial areas and the interference with other stations (commercial and private) very numerous and intense, making issues (often impossible) records of celestial radiation.
Further difficulty in reception at the lowest frequencies (such as to render the MF band hardly usable for amateur radio astronomy experiments) is caused by ionospheric interference that acts as a shield for the cosmic radiation, even if, in periods of minimum solar activity and in areas of the earth placed at high magnetic latitudes, the critical ionospheric frequency can reach values so low (overnight) to allow the sporadic detection of external radiation.
In the range of frequencies around 20 MHz, using relatively simple equipment such as those described below, can lead interesting observations on the decametric radiation of Jupiter that emits powerful, even if sporadic, noise "radio storms" generated by the interaction of charged energy particles with the magnetosphere of the planet. It can also groped recording solar "radio".
Even using not too directives antennas (at the limit, a simple dipole...) is not difficult to observe the radiation of the galactic center, repeating the experiences of Jansky, Reber and the first radio astronomers.
These experiences are particularly suitable for amateur experimenters, since it does not require the use of particularly expensive or complex tools and instruments to build and develop, or bulky antenna systems and difficult to install.
Any amateur can use its own receiving station in the HF band (if he can find a "slice" of spectrum free from interference), with yagi directional antenna (at least in azimuth) groped for the reception of the intense solar burst or Jupiter.
It is required a lot of patience, since the issues were sporadic and the location of a suitable receiving site, possibly located in places away from urban noise and artificial disturbances, it not seems, in our day, a trivial operation.
The receiver has to be equipped with an AM detection system and is indispensable disable the AGC (automatic gain control).
The achievable results, provided that you expected to schedule continuous observations for a period of enough long time (planning "listen" and adjustments of the signals present in the band), are very encouraging, as demonstrated by numerous experiences of fans who were able to obtain significant and interesting amount of data on the under study phenomena.
The following figure shows the recording of radiation at 20.4 MHz from the center of the Milky Way obtained by the direct-conversion receiver shown in the photo equipped with a simple half-wave dipole antenna with lobes oriented in NE-SW direction.
The experiment, developed with educational purposes for laboratory exercises in physics of a scientific institute, was intended to replicate the experience of Jansky, allowing students to understand and verify, in an elementary way, the problems associated with radio astronomy research.
The bandwidth of the receiver is of the order of 800 kHz, much wider than that which characterized the instrument of Jansky, centered on the frequency of 20.4 MHz.
The output data of the receiver were processed by a PC using a capture card with 8-bit ADC and integrating the detected signal with a time constant of the order of 16 seconds. The recording has started after about an hour of switching power, reaching the thermal stabilization of the system. Only at this point you have completed the adjustments of tuning. The BF and RF gain controls of the receiver are set to the correct level of sensitivity, so as to avoid unwanted phenomena of saturation.
Each graduation on the abscissa axis of the entry corresponds to a time interval equal to about one hour, while the numbers on the ordinate axis represent levels of relative intensity (have not performed any calibration procedure for the equivalent temperature of the incident radiation noise). Apart impulse noise due to local interference, are easily distinguished the large maximum periodicals produced by total radiation from Cygnus, Sagittarius (galactic center) and Cassiopeia. The emission has a maximum at around 03-06 o'clock and is rather widespread because of poor directivity of the antenna used (a simple dipole) that receives radiation from the horizon to the zenith without possibility to discriminate the various radio sources.
The simple system described is sufficiently sensitive and stable to pick up the galactic radiation and one of the main non-thermal radio sources, particularly intense at these frequencies (Cassiopeia, Cygnus...).
The quality of the observations depends on the antenna system used and from the place of installation of the receiving station, which must be particularly free from interference and from artificial disturbances.
In subsequent tests receiving of galactic radio wave, due to the large angular extent of the source, has been used with success a self maderow Yagi antenna with 3-elements arranged at a height of about 3 meters from the ground and, as a result, an Yagi commercial amateur antenna with 4-element modified to the operating frequency of 20.4 MHz.
Trial recording (transit) of the radio emission associated with the center of our galaxy carried out at a frequency of 26 MHz by Salvatore Pluchino (from Sicily) with the receiver to continues HF_RadioAstroLab tune, equipped with an Yagi antenna pointing to the transit at an inclination of 45° above the horizon.
Recording, as is typical when observations are made in the frequency band around 26 MHz, showing peaks related to interference (RFI) with other commercial radio stations and amateur radio. These disorders have not prevented, however, to highlight the profile of the transit of the radio source (this is the overall emission associated with the center of the galaxy).
The Jupiter radio system - Io
One of the most interesting planets for amateur radio astronomy studies, is Jupiter.
Most of the radiation emitted by the Jovian planetary system is much more intense, at the decametric frequencies, compared to that which could be explained by simple thermal mechanisms and is characterized by elliptical and circular polarization. Are involved mechanisms of radiation different from that heat, similar to those which take place in galaxies: it is synchrotron emission produced by charged particles (electrons and ions) accelerated by spiral lines of force of the magnetic field of the planet.
The thermal component of the radiation is of Jupiter, on the other hand, measurable using good amateur tools (characterized by sufficient effective area) operating in the microwave region.
In 1955, during a survey of the sky at a frequency of 22 MHz, B. Burke and K. Franklin recorded an intense and fluctuating noise coming from declination +22 degrees which lasted several days. The position of the radio source coincided with that of Jupiter: further observations confirmed that this planet was capable of emitting an intense and sporadic electromagnetic radiation in the range of decametric waves (HF), characterized by circular or elliptical polarization.
The equivalent temperature of the black body emission was of the order of millions of kelvin degrees, too high to be of thermal origin (the thermal radiation is not polarized). The thermal component of the radiation of Jupiter was measured in 1956 at a frequency of 10 GHz, showing an equivalent temperature of 140 K.
By subsequent observations at different frequencies, have characterized the overall basically radio emission of Jupiter as a combination of two components: the thermal, dominant at short wavelengths, the non-thermal, at long wavelengths. In the latter case it appears that the satellite Io has a very important role in the "modular" decametric radiation of the planet, as is shown by the figures that represent schematically the "Radio Transmitter" Jovian.
It can be relatively easily recorded with the radio-burst of Jupiter using a HF receiver tuned around the frequency of 20 MHz, connected to a simple dipole antenna folded (half wave) aligned in east-west direction and at the height of from the ground equal to a quarter wavelength, as shown in the following figure.
The receiver must be of the type to amplitude detection (AM) without automatic gain control circuit (AGC) which would act as a source of error in the evaluation of the intensity of the signals. Since the bandwidth of the received signals is relatively broad (of the order of one hundred kHz), it is not of primary importance (at least in a station amateur) the requirement on frequency stability in the short term, while it is very useful the possibility of easily change the tuning of the receiver to avoid interference from commercial radio stations and amateur radio.
To see if the receiving system is sufficiently sensitive for the study of radio-burst of Jupiter (more so for the study of the Sun, more intense), we recommend the following test: with the antenna connected to the receiver is tuned to a free stations frequency by adjusting the audio volume until you hear a loud blast of noise.
Remove the antenna cable by inserting in its place a resistive load (impedance equal to the antenna nominal): If the device is enough sensitive you should notice a significant decrease in the background noise, while otherwise it will be appropriate to include a RF preamplifier-preselector between antenna and receiver (narrow band to avoid intermodulation phenomena and to minimize interference caused by adjacent radio stations).
The following links are related to projects on the study of decametric radio emission of Jupiter, particularly suitable for amateur radio astronomers experimenters. The material includes interesting and deepened advice on equipment building economically accessible to amateurs.
Solar activity and geomagnetism, radio-sun and associated ionospheric effects
The Sun is one of the first objects studied by radio astronomers, not so much for its particular emission characteristics, as its proximity to the Earth which makes it very "bright" in the visible and in the radio band.
There are many electromagnetic phenomena that are originated in the Sun, phenomena that can be studied without much difficulty by willing and motivated enthusiasts.
Simplifying the phenomenology, we classify solar radio emission in three main components:
- Thermal component of the "quiet Sun" (relating to a minimum period of sunspot activity), always present.
- Slowly varying emissions.
Members of the "active Sun" caused by the activity of sunspots and flares.
The last two components are linked to the activity of sunspots: the slowly varying, of thermal origin, comes from the regions of the disc above the spots where are the highest electron density. The temperature in these regions exceeds the two million degrees, contributing to significantly increase the average level of emission associated with the radiation of the "quiet Sun", with slowly variable and proportional intensity to the number of spots on the disk (the flow radio linked to this mechanism follows the eleven-year cycle of sunspots).
The map of the radio emission due to solar flares (see figure) is much larger than that occupied by sunspots.
Result of a flare on the Sun's surface is a strong storm (burst) of electromagnetic energy projected into space.
It is possible to measure the radio component of these emissions using commercial receivers for HF and VHF band capable of detecting amplitude modulated signals (AM) and equipped with simple antennas: solutions and proposals presented in this section are suitable for the purpose.
The Sun burst
The solar bursts are classified as:
- TYPE I: narrow-band short events that usually occur concurrently with continued emissions broadband. They vary from a few hours to a few days.
- TYPE II: emissions that occur with a slow spectral drift of the frequencies from the top to the lowest. Often show a structure characterized by a fundamental and a second harmonic frequency.
- TYPE III: spectral emission characterized by rapid drift from high frequencies to low frequencies. In many cases highlight harmonics and are often accompanied by rapid "flash" phase due to intense bursts of electromagnetic energy (flare).
- TYPE IV: continuous emissions broadband associated with flares (flare).
- TYPE V: continuous wide-band emissions which may appear along with bursts of type III. Have a duration equal to about 1 or 2 minutes, increased with decreasing frequency.
The activity on the Sun surface is shown by the density of sunspots that appear as dark areas on the photosphere, floating in frequency within a cycle of activity approximately equal to 11 years. They are dark regions because more "cold" respect to the bottom: their temperature is of the order of 4000 K, while that of the surrounding surface is 6000 K.
In sunspots are localized intense magnetic fields and, on the part of the immediately above atmosphere, often occur intense flares (flare) that produce powerful burst of radio energy at frequencies between about 5 MHz and 300 MHz.
Often, during the more intense flares, is emitted an intense flow of charged high energy particles (cosmic rays) traveling at a speed of 500-1000 km/s: When these particles reach the earth's magnetic field they are due to intense radio disturbances and magnetic storms with the formation of auroras.
Unlike the radiation coming from the majority of the celestial radio sources, which is not polarized, that associated with solar flares is circular polarization, being caused by the spiral trajectories of electrons that follow the local, intense magnetic field associated with the blasting.
Radioastronomy studies are conducted on the Sun by direct observation, observing and recording the effects of solar radiation on the Earth's ionosphere.
A magnetic field can be mathematically represented by a vector, set of numbers representing the amplitude and direction of the variable at each point of a surface or a volume.
The Earth's magnetic field is generally described by amplitude and two angles that determine its orientation, known as inclination and declination. Some measuring instruments are sensitive to only some components of the magnetic field, as in the case of the compass indicating the horizontal orientation (declination), but not the vertical (inclination) or amplitude.
The spatial distribution of terrestrial magnetism is generally approximated with that which would be produced by a short and intense dipole located near the center of the Earth and shifted respect to the axis of rotation. The introduction of additional dipoles (characterized by lower intensity and with different orientations) allows you to refine the model and adapt it to local conditions.
The field measured at the earth's surface, in addition to not being constant over time (with long-term oscillations of the order of years), is sensitive to the distribution of local mineral deposits and interacts with the particles electrically and magnetically charged from the Sun.
The Earth's geomagnetic activity is a phenomenon induced by the solar wind, flow consists of a plasma of free electrons and ions ejected from the Sun: in the presence of strong disturbances on the star, the solar wind can significantly increase the speed of its interaction with the Earth's magnetosphere altering the distrubution and producing abrupt and sudden changes of the field.
Also, looking for daily changes due to the combined action of solar heating that induces circulating electric currents in the upper atmosphere, and the tide effect caused by the sun's gravitational that redistributes the ionospheric plasma.
Such variations are adjustable (although with much less intensity) produced by the tide effect of the moon that causes weak ionospheric currents.
Other non-periodic variations are produced by physical phenomena identical to those responsible for the solar wind at specific areas of the crown, called coronal holes, the local magnetic field allows a free output of the solar wind. Some particles "brush" the Earth's magnetosphere, in response, undergoes expansion and contraction.
Solar flares are sources of intense electromagnetic radiation, X-rays, ultraviolet radiation and charged particles (electrons, protons, heavier kernels). Such radiation increases the ionization percentage of the Earth's ionosphere, while the particles are generally diverted from the earth's magnetic field. Broader distortions of Earth's magnetosphere are caused by coronal mass ejections.
The intensity of the total magnetic field measured on the earth's surface varies between 0.1 and 1.0 Gauss (0.00001 and 0.0001 Tesla), generally directed towards the magnetic poles with predominant horizontal direction toward the tropics and mid-latitudes, and is inclined, in the vertical direction, toward the polar regions. There are temporal smaller variations of two or more orders of magnitude during the day and the night, on time scales ranging from a few seconds to several hours (up to a whole day).
An indirect method is widely used to monitor solar flares, then reveal the so-called ionospheric effects, provides for the permanent monitoring, in VLF band (typically at frequencies below 150 kHz), of a strong and stable signal from a far enough radio station, recording the intensity variations of emission over time.
The following material is a "radio sun" fascinating topic, including the study of solar radio burst, the receipt and recording of electromagnetic events (the lower limit of the window radio) associated with the activity of the solar disk, to illustrate the Induced electromagnetic phenomena on the Earth's ionosphere.
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