UR: Shot Noise in the Dark: Using Parker Solar Probe Current-Biased Antennas

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Ember Winry

UC Berkeley Space Science Laboratory

Winry Ember is a junior specialist working at UC Berkeley’s Space Science Laboratory during her gap year. She is working with Dr. Marc Pulupa to study the properties of heliospheric plasma using data from NASA’s Parker Solar Probe. When she’s not analyzing plasma data, she enjoys crocheting, playing the harp and leading physics-themed yoga classes.

Image of the solar corona during a total solar eclipse Monday, August 21, 2017 above Madras, Oregon. Credit: NASA/Aubrey Gemignani

NASA’s Parker Solar Probe (PSP), launched in 2018, is a groundbreaking heliophysical mission to study the Sun’s atmosphere. The PSP became the first spacecraft to ‘touch’ the sun when it entered the solar corona – the outermost part of the solar atmosphere – in April 2021. While this pioneering probe provides extraordinary insights into our host star, collecting precise scientific data in such a unique environment can be quite a challenge, even when everything is working properly on the spacecraft. Let’s explore some of the measurements taken to more accurately assess the plasma data collected by the spacecraft’s Fields Experiment Instrument Suite (FIELDS), which measures electric and magnetic fields around the sun.

Plasma is a gas that has been ionized or heated to such extreme temperatures that individual electrons are separated from the atoms of the gas. Gas atoms become ions when they lose their negative charge, and we are left with a gaseous soup of negatively and positively charged particles – or plasma. Because these free electrons and ions are always attracted to each other, they oscillate around each other at a natural frequency. The denser the plasma, the stronger the force bringing them together will be, resulting in a higher plasma frequency. Using data from the FIELDS radio frequency spectrometer (RFS) and a technique called quasi-thermal noise spectroscopy (QTN), we can determine the densities and temperatures of electrons in the heliospheric plasma.

A sketch of the Parker Solar Probe with the FIELDS antennas shown
Illustration of Parker Solar Probe with FIELDS antennas indicated by orange arrows
Credit: The Spectator/Aries Ho

QTN analysis uses measured electric field spectra to examine the location and shape of a plasma’s frequency peak (Fig 1). Because QTN provides exceptionally reliable and accurate measurements of the electronic parameters of a plasma, this technique is commonly used to calibrate other instruments on PSP. With so many scientists relying on QTN measurements for proper instrument operation, it is important that our tuned electron parameters are as accurate as possible.

Figure 1. Example of QTN spectrum (Ulysses/URAP data in the solar wind), Meyer-Vernet et al 2017. The electron temperature is calculated by examining the slope of the plasma line, and the fp peak reveals the total electron density.

Running QTN with an instrument that’s part of a larger suite of instruments is a bit like having roommates, though – there’s plenty of opportunity for growth and collaboration, but we have to be prepared to make compromises. In the case of electric field measurements on PSP, such a compromise is between the high frequency RFS spectra and the low frequency electric field measurements made using the same FIELDS antennas. Accurate measurements of low frequency electric fields in the solar wind require “current biasing”, a technique where a current is applied to the electric field antennas in order to bring the electric potential of the antennas closer to the potential of the plasma. Adding a bias current generates electrons that change the antenna voltage as they exit. The high frequency signature of these small voltage changes is called shot noise and makes QTN analysis more difficult by masking the plasma line and frequency spike. Therefore, our goal is to quantify the effects of this shot noise on RFS data, and thus improve QTN measurements of electron density and temperature in the inner heliosphere.

The best place to start was to analyze data from a regular instrument calibration activity called a bias scan. During a bias scan, the applied current of the antenna varies rapidly over a wide range from negative to positive currents, allowing us to see how the plasma line responds to various applied currents, including the current of zero bias (Fig 2A). Then, by subtracting the average zero bias line from each bias scan spectrum, we are able to isolate the effect of current variation (Fig 2B).

Figure 2. (left) Spectra from various bias scan intervals. The average plasma line with no current applied (ideal QTN conditions) is displayed in dark blue. The antenna voltage of -12 milliamps, shown in red, visualizes a line of plasma from our typical applied current.
(right) Spectral slices with zero bias line subtracted

Another way to visualize the variation of RFS data with bias current is to draw a section through the spectrum at a given frequency with the antenna current on the x-axis (Fig 3). This graph reveals that the shot noise generated from our nominal bias occurs where we observe the plasma frequency peak and, more importantly, shows that it is possible to measure and understand the effect of the bias of running.

Picture 3. Power vs frequency cutoff taken at 100 kHz. The frequency of shot noise visibly decreases as the current applied to the antenna approaches zero.

In the future, we plan to compare data from hundreds of other bias scans performed on PSP and fit the generated spectra to the power law models predicted by theory. Because PSP is the first spacecraft to conduct QTN on a current-biased antenna, this is a unique problem that is not fully described by contemporary antenna response models. By better documenting some of the unique challenges of collecting and modeling in situ solar corona data, we hope to extend current QTN analysis models to account for operation with bias current. While the PSP offers heliophysicists an unprecedented opportunity to explore the solar corona, none of this matters if we can’t make precise measurements with the data it collects. This can often be a challenge, but it’s nothing a little plasma physics and elbow grease can’t solve.

Astrobite edited by: Sahil Hegde

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