HDAC status and analysis: Photometric observations by HDAC onboard Cassini Yuri Skorov, Horst Uwe Keller, Ralf Reulke, Karl-Heinz Glassmeier, Vlad Izmodenov, Olga Katushkina

2008 Saturn position from 2001 to 2017 relative to the upwind direction of interstellar hydrogen (arrow). For 2008 observations obtained in rotational mode give us instant scan in a plane parallel to the direction of hydrogen flow.

“Catastrophic” events in 2001 Overview of HDAC photometer data for 2001 and 2002.

SPICA observations: Off Axis Response HDAC off-axis response. SPICA Raster Scan 2002 The off-axis response was measured in the laboratory (red line). Blue line – fitting curve for the raster scans, green circles – one of the scans. . How stable is this 2D map of off-axis response? We have raster scans for 2002 and 2003 and many single-pass observations in 2002-2012.

Raster Scan. Line of Sight movement Raster Scan. Central part of off-axis response map. 2D inhomogeneities are clearly visible Raster Scan. SPICA data set

Pseudo normalized SPICA observations 2006-2011 Pseudo normalized SPICA observations 2006-2011. The data from HDAC2006_037_23_32_06 was used for normalization. Observation time is 0.125 s. Scan directions of SPICA observations 2006-2011. Angle differences between the star position and the line of sight.

Sensitivity variations across the FOV (off-axis response) are very stable. Angular distribution of sensitivity is not axisymmetric. Central noisy part of FOV plays only minor role for analysis of LISM hydrogen observations.

HDAC calibration via SPICA observations HDAC relative spectral sensitivity (UVIS Calibration Report) Spica spectra for 1979-05-06 (https://archive.stsci.edu/iue/). Flux variation over one day is clearly visible: the first spectrum is obtained at 18:15:46, the last one at 23:28:10.

Total FUV signal with linear trend (due to sensitivity decline) divided out. Blue line is the predicted variation in flux from the model (Greg Holsclaw, team meeting presentation). Spica spectra (black curve) and its product with the spectral sensitivity function (red curve).

Sensitivity decreases very slowly. Today 11 cts/s/R. Absolute HDAC sensitivity in counts/s/R evaluated from the Spica observations (blue curve) and LISM observations (red curve).

IPH observations 2011: rotational mode Star Fixed orientation Hydrogen observations in 2011. Rotational mode. Integration time 4 sec. Red curves: HDAC2011_002_00_41_24_4.0, green curves: HDAC2011_058_21_03_20_4.0.

FUV and HDAC

Examples of spectra and extrapolation IPH observations 2011: rotational mode Examples of spectra and extrapolation

Example of obtained preliminary results IPH observations 2011: rotational mode HDAC2011_058_21_03_20 Example of obtained preliminary results HDAC2011_124_06_18_15

Distribution of UV objects in the sky (from ESRO TD1 catalog)

IPH observations 2011: rotational mode

IPH observations 2008: rotational mode During 2008-2009 NASA scientists noted that the Sun is undergoing a "deep solar minimum," stating: "There were no sunspots observed on 266 of [2008's] 366 days (73%). Prompted by these numbers, some observers suggested that the solar cycle had hit bottom in 2008. /wikipedia.org/

3D model of the hydrogen distribution inside the heliosphere (Izmodenov&Co) Boltzmann kinetic equation: m=m(t,l,vr) rE=1 AU TS LISM LISM Hydrogen distribution in the heliosphere is effected by: - Local effects that are important near the Sun: (solar gravitation Fg, radiation Frad and ionization bE) - Global effects: distribution function of H atoms is distorted in the heliospheric interface due to charge-exchange => distribution function is not Maxwellian at 90 AU (Izmodenov et al., 2001) Simple self-absorption approximation for the calculation of the Lyman-alpha spectra is used (no multiply scattering) fondue Outer boundary is at 90 AU

Comparison: raw data vs. model model data Comparison: raw data vs. model Time interval is only 7 days 2004/220 2004/22 2004/13 Larger amplitude Smaller amplitude Time Time Time

Comparison: raw data vs. model Observations (red curve) for DOY 2.2011. The abscissa is time in simulation steps. Integration time is 4 seconds. The ordinate is dimensionless intensity of Lyman alpha emission (observation and theory). Dashed red curves = observations +- 2σ. Dark blue curves correspond to different ionization rate: solid/dashed=5. Light blue curves correspond to 3D time dependent models: solid curve for self absorption model, dashed curve for optically thin model. DOY 2 2011 HDAC Time

CONCLUSIONS Sensitivity of HDAC is sufficient Observations in the rotational mode allow to obtain unique data on the 3D distribution of hydrogen A preliminary comparison of the data with theoretical models shows that the accuracy of the observations is sufficient to verify the models We must perform an extensive comparison of data from HDAC and FUV to improve the absolute calibration