How to make canvas - shadow on a translucent shape?

Question

Canvas

I have a translucent form:

this.cx.beginPath();
this.cx.globalAlpha = .1;
this.cx.fillStyle = gradientFill;
this.cx.strokeStyle = gradientStroke;
this.cx.arc(150, 150, 139, 0, Math.PI * 2, true);
this.cx.lineWidth = 1;
this.cx.stroke();
this.cx.fill();

I want to add some shadow, but I only want it to appear outside of the form, I think more of a glow than a shadow. Is there a way to do this in canvas as my attempts are:

this.cx.shadowColor = 'rgba(0, 0, 0, .75)';
this.cx.shadowBlur = 5;
this.cx.shadowOffsetX = 5;
this.cx.shadowOffsetY = -5;

A dark shadow is visible through the translucent form.
Thanks to!