Why do High-Voltage devices spark at high altitude?

[Lunar_Lander]

Hello,

I have moved that question around in my mind for quite a while now. I have gotten several datasets from amateur high altitude balloon flights in which the Geiger Counters onboard delivered either no data or excessive count rates, and this occurred almost every time when a balloon crossed a certain altitude on the ascent (for example 22 km) and the GCs worked again when the balloon came back down below this altitude.

Thus I remembered a text on the high altitude research rockets in the 1950s, in which it was said that high-voltage devices for the rockets posed a problem because in the near vacuum, the HV devices would spark. My idea now is that this sparking also occurred in the balloons.

But the question now is: Why does HV spark in low pressure? As well as the pressure, density decreases with altitude, thus there should be less particles to conduct a spark, on the other hand air normally is an isolator, and atmospheric electricity is caused by Ions.

I would be happy if someone could help me with that question!

Thanks,
L_L

 

[uart]

As well as the pressure, density decreases with altitude, thus there should be less particles to conduct a spark

At normal atmospheric pressure the air molecules actually inhibit the spark, as electrons usually collide with an air molecule before they have enough energy to cause ionization.

At lower pressure/density the electrons have a larger free path length and thus gain more energy before colliding.