The Solar Orbiter ESA mission

Solar Orbiter is a mission dedicated to solar and heliospheric physics. It was selected as the first medium-class mission of ESA’s Cosmic Vision 2015-2025 Programme. The spacecraft was launched on February 10, 2020 from the Kennedy Space Center (KSC) in Florida.

The Solar Orbiter mission aims to make significant breakthroughs in our understanding both of how the inner heliosphere works, and of the effects of solar activity on it. The spacecraft takes a unique combination of measurements: in situ measurements will be used alongside remote sensing close to the Sun to relate these measurements back to their source regions and structures on the Sun’s surface. It operates both in and out of the ecliptic plane. Solar Orbiter measures solar wind plasma, fields, waves and energetic particles close enough to the Sun to ensure that they are still relatively pristine.

The top level science questions of the mission are :

• How and where do the solar wind plasma and magnetic field originate in the corona?
• How do solar transients drive heliospheric variability?
• How do solar eruptions produce energetic particle radiation that fills the heliosphere?
• How does the solar dynamo work and drive connections between the Sun and the heliosphere?

More information can be found on the ESA Solar Orbiter website.

The Radio and Plasma Waves instrument (RPW) on-board Solar Orbiter

The RPW instrument consists in a sophisticated plasma/radio wave receiver system connected to high sensitivity electric and magnetic sensors (antennas and SCM), as can be seen on the diagram below. A detailed description of RPW can be found in Maksimovic et al., The Solar Orbiter Radio and Plasma Waves (RPW) instrument, Astronomy & Astrophysics, 642.

The receiver system covers a very wide frequency range (quasi-DC to 20 MHz for electric, and 0.1 Hz to 1 MHz for magnetic) thanks to several subsystems described below.

RPW main sub-systems

The Bias unit

BIAS unit is responsible for (1) amplifying and distributing the analog signal from antennas in the LF frequency range, and (2) driving the current bias to antennas in order to perform high quality DC measurements. More

Thermal-Noise Receiver – High Frequency Receiver (TNR-HFR)

TNR-HFR consists in a broadband and high-resolution spectrometer integrating a double channel radio receiver in interface with the three monopoles for the electric field and the high frequency component of the magnetic search-coil. The TNR produces quasi-instantaneous spectra for the electrostatic thermal noise and/or the magnetic field in the range from 4 kHz to 1MHz. The HFR is a sweeping receiver, in the range from 500 kHz to 16.4MHz. More

Low Frequency Receiver (LFR)

LFR is designed to produce and transmit waveforms (WF), averaged spectral matrices (ASM) and basic parameters (BP) from LF electromagnetic data (quasi DC-10kHz). More

Time Domain Sampler (TDS)

TDS is a medium frequency wave analyser module. It allows digitizing and processing analog signals from electric field antennas and search coil at sampling rates up to 512 kHz.
Its primary function is to provide on-board selected multi-component waveform snapshots at high sampling rate. Additionally, TDS allows to sample low frequency electric and magnetic field inputs. More

RPW science data acquisition modes

RPW flight software can operate using two main science data acquisition modes: survey and selective.

Flight software mode is commanded from ground by the RPW team.

Survey modes

In the survey mode, RPW analysers acquire science data on-board continuously. The survey data telemetry is then automatically transferred by the instrument into a dedicated packet store in the spacecraft solid state memory, which is regularly flushed by ESA, depending of the telemetry downlink rate available at each pass.

The data acquistions can be performed using pre-defined normal or high rate samplings, respectively named SURVEY_NORMAL and SURVEY_BURST submodes. Even in these two submodes, RPW analysers remain however widely configurable.

Detection mode

In the detection mode, RPW samples the TDS and LFR signals in a very high rate in parallel to the normal cadence data acquisitions. Contrary to the survey mode data, these very high rate data samples are not systematically downlinked on-ground, but remained on-board. The high rate data are first stored in a dedicated rolling buffer in RPW memory. They are automatically dumped by the RPW flight software (i.e., transferred to a dedicated telemetry packet store in the spacecraft solid state memory) under conditions only, which are ruled on-board by a given detection algorithm. The interesting enough high rate data can be then downlinked on ground by ESA on the RPW team request. Since the packet store for the "detection mode" has a limited data volume, this "selective downlink" cycle must be performed in a limited duration before the high rate data for a given detection are overwritten.

There are two detection submodes:

• A first "Selected Burst Mode 1" (SBM1) to release data from buffer when in situ interplanetary shocks are detected.
• A second "Selected Burst Mode 2" (SBM2) to release data from buffer when in situ langmuir waves associated with type III solar radio burst electron beams are detected.
  Both submodes are run in parallel when RPW operates in the detection mode.

Some input parameters of the SBM1 and SBM2 algorithms can be set by command, in order to optimize the detection of shock and langmuir waves respectively.