Radio Heliophysics

Radio heliophysics is the study of the sun at radio frequencies. Since the level of noise in the longwave radio wavelength bands on Earth is so high, only limited studies of the sun have been made at these wavelengths. LUNAR is conducting research to refine the design of a lunar solar radio telescope that could be deployed on the near side of the moon, either robotically or by astronauts on a sortie mission. The current design is called the Radio Observatory for Lunar Sortie Science (ROLSS). The LUNAR team is improving this design by analyzing existing low frequency radio observations to quantify the solar, lunar, terrestrial, and transient sources of radiation at these frequencies. The team is also working on the antenna design, simulations of the response of the instrument to the Sun and the low frequency sky, and on methods for deploying antennas on the lunar surface.

Scientific Motivation

The high temperature solar corona produces the supersonic solar wind that creates a magnetic bubble around our solar system called the heliosphere.  Over the course of the eleven-year cycle of solar activity the heliosphere changes.  These changes include violent solar flares and coronal mass ejections, which can create high-energy radiation and magnetic disturbances that, can effect communications, navigation, and human safety.  The goal of the Heliophysics Key Science Project is to conduct low-frequency radio science of the heliosphere from the surface of the moon in order to determine how the Sun accelerates particles to high energy.  This work is divided between investigations of radio emission from space, designing a solar imaging radio array, and developing new technology to make radio astronomy on the lunar surface possible.  

Theoretical tools

In the first year of LUNAR research, the investigations of radio emission included characterization of the lunar radio frequency interference (RFI) environment and a search for low frequency radio transients. This was done using existing observations in deep space and near the Moon.  As the first component of our analysis of the lunar RFI background, the team analyzed Wind Waves RAD2 radio data (frequency range 1-14 MHz), acquired from November 1994 through the present. The goal was to measure the intensity level of radio bursts and terrestrial emissions observed by the Wind spacecraft along its complex trajectory, which includes passes very close to the Moon.  While these data demonstrate that successful radio observations of cosmological sources will be best accomplished on the far-side of the Moon, they also indicate what might be accomplished from the near side. 

Solar radio bursts are another potential target for the lunar radio telescope observations; the most intense of the solar radio emissions in this band is the type III burst, but covering less area in frequency-time space. Our requirement for the observational portion of this task has been for the 32 antennae (32T) prototype to observe solar radio bursts. LUNAR teammembers have begun analyzing the same dataset of these radio bursts with different numbers of antennas.  LUNAR also looks at data collected at night during the same period to evaluate how such a small array can produce a detailed sky survey.  Results of this part of the task are expected in year three.

LUNAR continues to study the number of antennas needed to successfully image solar radio emission.  The more antennas there are and the larger the separation between them, the higher quality of the resulting images will be, in terms of both angular resolution and point spread function and dynamic range.  However, a large number of antennas leads directly to higher resources required for the array, and mass will likely be the largest single constraint on deployment.  LUNAR is studying both how the quality of solar images depends on the number of antennas, and developing software and analysis techniques to improve the quality of the data per antennae.  Work on this topic is based on the Murchison Widefield Array (MWA), specifically a prototype system consisting of 32 antennas built in Western Australia.  The 32T prototype has a 300m baseline and observes from 80-300 MHz.  This is very close to the number of antennas under consideration for a lunar solar array.  While the 32T prototype operates at slightly higher frequencies than a lunar array, the antennas are more closely spaced, resulting in very similar angular resolution and image quality.

In parallel with the analysis of observations, LUNAR has developed software algorithms to form higher quality images.  The focus has been in developing ways to subtract the bright unpolarized emission from the Sun in order to prevent contamination from solar bursts or from other objects in the sky. These techniques will be essential for both a solar array on the nearside of the moon, and for concepts for larger cosmology-focused arrays on the farside. Low frequency arrays inherently have large fields of view, and must be able to cope with bright sources anywhere in the sky.  Teammembers have produced an eigen value method for extracting the solar contribution to images in real time.  LUNAR is currently evaluating the technique with the MWA data.

Technology Development

The technology development task for the Radio Heliophysics Key Project consists of demonstrations of deployment techniques for a lunar radio array, testing of radio antennas designs, and work on low temperature low power electronics. The radio-heliophysics effort focused on advancing the readiness of our proposed Radio Observatory on the Lunar Surface for Solar studies (ROLSS).  The ROLSS concept uses dipole antennas deposited on rolls of Kapton® film, which would be compactly stored for flight and easily deployed on the lunar surface by unrolling.  

Throughout the LUNAR team, activities are taking place to advance the technologies required to make ROLSS or larger lunar radio arrays a functioning reality. One concern about the original concept for ROLSS (Figure 1) is that the antenna-transfer line system was passive, i.e., there was no pre-amplification at the antennas in order to avoid having to provide power to such components.

 

Figure 1:  Artists depiction of the ROLSS concept (Radio Observatory on the Lunar Surface for Solar studies) sitting on the lunar surface. This array would be comprised of single radio dipoles embedded within polyimide kapton film.  Each antenna would be roughly 500 meters in diameter.

Amplification would occur when the signals arrived at the Central Electronics Package, but the losses from the antennas 500 m away would be substantial – up to 25 dB.  While this might be acceptable for the more intense solar radio bursts, it is clearly problematic for weak signals, such as those of cosmic origin.  

To deal with the resulting signal loss, a concept of active components on the film at or near the individual antennas is being studied.  A key concern for ROLSS, which only operates during the lunar day, is whether such components survive the thermal cycling imposed by the Moon’s rotation.  For cosmic radio observatories planning to observe at night, the question becomes whether the components will function at lunar night temperatures, or how much power is required to heat them. 

As the first step in solving these problems, teammembers tested the performance of a Texas Instruments (TI) 16-bit ADS5483 analog digital converter (ADC) from room temperature to -250° C. Such an ADC would likely not be used for the interferometric data, given the excess number of bits calculated. It can be used as the total signal amplitude monitor, measuring the full range of solar and other radio signals received at the ROLSS site.  

The parameter used to describe the quality of the ADC function is the “effective number of bits” (ENOB). The noise of the test system reduced the measured ENOB at room temperature to ~10 bits, as shown in Figure 2.  This level of performance was maintained down to -200° C, below the typical lunar night surface temperatures.  The next experiment will cycle the ADC repeatedly over thermal ranges analogous to the change in lunar surface temperatures near the lunar equator to determine survivability.  

Figure 2: Data collected during the thermal cycling experiment on the Analoge Digital Converter proposed to measure the range of frequencies observed by the ROLSS antennae.  

Although ROLSS may be thought of as a small lunar radio observatory, the new design elements , in particular, the antennatransmission line system deposited on polyimide film, would need to be be tested by a pathfinder mission.  Therefore, LUNAR is developing a small, lightweight package that could fly as a secondary payload on an unmanned lunar lander. This lander would deploy ~50 m of film with two dipole antennas and transmission lines for testing the array concept. LUNAR is considering a partnership with a Google Lunar X-Prize team to deploy this small pathfinder array from their payloads.

A number of options have been considered for the deployment mechanism of this small arm of Kapton film. The current efforts focus on a system where an anchor is spring-launched to >50 m and the line through the anchor is pulled in by a small motor, thereby pulling out the film.The critical element is the anchor, which must hold adequately on the range of surfaces anticipated. An anchor and pulley system that can hold on dusty or rocky surfaces has been proposed. The team is now developing a testable model for a vacuum chamber experiment.

Figure 3: A pulley and anchor system designed to be easily deployed from a simple lunar lander. The anchor is shot out of the payload, and a strip of film is then unwound by a string running from the payload to the anchor. 

 

Potential New Key Project: Interplanetary and Interstellar Dust

For a long time, dust has been known to exist in interplanetary and interstellar space in the micro-meter size range, through the remote observation of the zodiacal light produced by sunlight scattering through interplanetary dust particles.  Since the first space exploration missions, in-situ dust detection has been an important observational goal. Most of these measurements have been performed with instruments specifically designed to characterize dust particles.  In the last several years several new studies have shown that radio receivers on spacecraft are be able to (although not designed to) measure electric signals associated with individual dust grains impacting the spacecraft body at high speed.  This discovery was based on an analysis of dynamic radio spectra measured by the WAVES electric field antennas on the STEREO spacecraft, which showed that individual dust impacts on the spacecraft produce a strong signal at frequencies below 1 MHz.  The STEREO spacecraft was launched in 2008, and these results have appeared in the literature in the last few years.  

LUNAR teammembers realized that if dust produces a detectable signal in a radio receiver in space, this could potentially open an entire new field of scientific investigation for a lunar radio array.  The array would have a significantly larger collecting area than an antenna on a single spacecraft, and could therefore make much more sensitive surveys of the distribution of interplanetary dust as a function of mass.

If the individual signals produced by dust impacts produce a distinct signal, LUNAR can optimize the lunar array to improve the accuracy of the derived dust properties.  The team needs to characterize the rate and intensity of dust signals to ensure that there will be no impact to our primary science goals. The team used measurements from the two STEREO spacecraft to investigate three questions: How much information can be derived about individual dust impacts if the actual electric field waveforms are collected instead of the power spectra reported previously? Over what range of masses can a space-based radio array study dust? What would the event rates be for a lunar radio array due to dust impacts, and is that a scientific opportunity or a threat to the other observation plans?

The team concludes that a low frequency radio receiver such as the S/WAVES instrument on STEREO, or a future lunar radio array, can be used as a dust detector. The data can range from dust sizes in the nanometer to the sub-micrometer range.  In the latter case, the orbital motion of the spacecraft can distinguish between interstellar and interplanetary dust components. These results reasonably agree with previous studies and with current dust flux models. They suggest that a lunar radio array could successfully add the sources, variability, and mass distribution of interplanetary dust as a key science goal.  On the basis of this study, there are three recommendations for ROLSS.  The addition of an automated dust counter that registers the amplitude and time of each impacts would greatly reduce the error bars on the flux measurements, especially in the nano-size range where the fluxes are high.  A larger dynamical range (S/WAVES TDS saturates at  170 mV ) would enable ROLSS to scan a larger (and different) mass range.  The use of well-separated antennas in ROLSS would gather more information about single-hit dust impacts, specifically determining the dust grain velocity vector.

Members of the LUNAR radio heliophysics key project at SAO include Justin Kasper, Lincoln Greenhill, Jonathan Weintroub, and Peter Cheimets. Other members of the team are Robert MacDowall (NASA/GSFC), Stuart Bale (UC Berkeley), Tim Bastian (NRAO), and John Grunsfeld (NASA/JSC). We are also working closely with Joseph Lazio (NRL), Jackie Hewitt (MIT), and Chris Carilli (NRAO), who are working on the LUNAR low frequency astrophysics and cosmology key project.